For my data import I suddenly get the following error:

neo4j.exceptions.DatabaseError: NOT PART OF CHAIN! RelationshipTraversalCursor[id=328769435, open state with: denseNode=false, next=328769435, mode=regular, underlying record=Relationship[328769435,used=false,source=0,target=0,type=0,sPrev=0,sNext=0,tPrev=0,tNext=0,prop=0,secondaryUnitId=-1,!sFirst,!tFirst] ]

What does this error actually mean? And how could I debug it further?

I haven't found any documentation on this.

last part of the debug.log (more not allowed by StackOverflow):

2018-07-19 14:29:57.551+0000 ERROR [o.n.b.v.r.ErrorReporter] Client 

...
org.neo4j.cypher.internal.runtime.slotted.pipes.ApplySlottedPipe$$anonfun$internalCreateResults$1.apply(ApplySlottedPipe.scala:34)
        at scala.collection.Iterator$$anon$12.nextCur(Iterator.scala:435)
        at scala.collection.Iterator$$anon$12.hasNext(Iterator.scala:441)
        at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:409)
        at scala.collection.Iterator$class.foreach(Iterator.scala:891)
        at scala.collection.AbstractIterator.foreach(Iterator.scala:1334)
        at scala.collection.generic.Growable$class.$plus$plus$eq(Growable.scala:59)
        at scala.collection.immutable.VectorBuilder.$plus$plus$eq(Vector.scala:732)
        at scala.collection.immutable.VectorBuilder.$plus$plus$eq(Vector.scala:708)
        at scala.collection.TraversableOnce$class.to(TraversableOnce.scala:310)
        at scala.collection.AbstractIterator.to(Iterator.scala:1334)
        at scala.collection.TraversableOnce$class.toIndexedSeq(TraversableOnce.scala:300)
        at scala.collection.AbstractIterator.toIndexedSeq(Iterator.scala:1334)
        at org.neo4j.cypher.internal.runtime.slotted.pipes.EagerSlottedPipe.internalCreateResults(EagerSlottedPipe.scala:40)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:76)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.runtime.interpreted.pipes.PipeWithSource.createResults(Pipe.scala:72)
        at org.neo4j.cypher.internal.compatibility.v3_4.runtime.executionplan.BaseExecutionResultBuilderFactory$BaseExecutionWorkflowBuilder.createResults(DefaultExecutionResultBuilderFactory.scala:105)
        at org.neo4j.cypher.internal.compatibility.v3_4.runtime.executionplan.BaseExecutionResultBuilderFactory$BaseExecutionWorkflowBuilder.build(DefaultExecutionResultBuilderFactory.scala:77)
        at org.neo4j.cypher.internal.compatibility.v3_4.runtime.BuildInterpretedExecutionPlan$$anonfun$getExecutionPlanFunction$1.apply(BuildInterpretedExecutionPlan.scala:97)
        at org.neo4j.cypher.internal.compatibility.v3_4.runtime.BuildInterpretedExecutionPlan$$anonfun$getExecutionPlanFunction$1.apply(BuildInterpretedExecutionPlan.scala:80)
        at org.neo4j.cypher.internal.BuildSlottedExecutionPlan$SlottedExecutionPlan.run(BuildSlottedExecutionPlan.scala:152)
        at org.neo4j.cypher.internal.compatibility.LatestRuntimeVariablePlannerCompatibility$ExecutionPlanWrapper$$anonfun$run$1.apply(LatestRuntimeVariablePlannerCompatibility.scala:128)
        at org.neo4j.cypher.internal.compatibility.LatestRuntimeVariablePlannerCompatibility$ExecutionPlanWrapper$$anonfun$run$1.apply(LatestRuntimeVariablePlannerCompatibility.scala:124)
        at org.neo4j.cypher.exceptionHandler$runSafely$.apply(exceptionHandler.scala:89)
        at org.neo4j.cypher.internal.compatibility.LatestRuntimeVariablePlannerCompatibility$ExecutionPlanWrapper.run(LatestRuntimeVariablePlannerCompatibility.scala:124)
        at org.neo4j.cypher.internal.PreparedPlanExecution.execute(PreparedPlanExecution.scala:29)
        at org.neo4j.cypher.internal.ExecutionEngine.execute(ExecutionEngine.scala:119)
        at org.neo4j.cypher.internal.javacompat.ExecutionEngine.executeQuery(ExecutionEngine.java:61)
        at org.neo4j.bolt.v1.runtime.TransactionStateMachineSPI$1.start(TransactionStateMachineSPI.java:144)
        at org.neo4j.bolt.v1.runtime.TransactionStateMachine$State.startExecution(TransactionStateMachine.java:446)
        at org.neo4j.bolt.v1.runtime.TransactionStateMachine$State$2.run(TransactionStateMachine.java:343)
        at org.neo4j.bolt.v1.runtime.TransactionStateMachine.run(TransactionStateMachine.java:81)
        at org.neo4j.bolt.v1.runtime.BoltStateMachine$State$2.run(BoltStateMachine.java:456)
        at org.neo4j.bolt.v1.runtime.BoltStateMachine.run(BoltStateMachine.java:222)
        at org.neo4j.bolt.v1.messaging.BoltMessageRouter.lambda$onRun$3(BoltMessageRouter.java:93)
        at org.neo4j.bolt.runtime.MetricsReportingBoltConnection.lambda$enqueue$0(MetricsReportingBoltConnection.java:69)
        at org.neo4j.bolt.runtime.DefaultBoltConnection.processNextBatch(DefaultBoltConnection.java:193)
        at org.neo4j.bolt.runtime.MetricsReportingBoltConnection.processNextBatch(MetricsReportingBoltConnection.java:87)
        at org.neo4j.bolt.runtime.DefaultBoltConnection.processNextBatch(DefaultBoltConnection.java:143)
        at org.neo4j.bolt.runtime.ExecutorBoltScheduler.executeBatch(ExecutorBoltScheduler.java:163)
        at org.neo4j.bolt.runtime.ExecutorBoltScheduler.lambda$null$0(ExecutorBoltScheduler.java:145)
        at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:748)

This also happens now for files which I successfully imported before.

This must have to do something with my cypher query because it works when I reduce it to the absolute minimum, but it would be helpful to understand the error.

EDIT - TRYING TO DEBUG

Now I am quite stuck with this error.

With this super minimal Cypher query from my script everything works fine:

WITH $custom_dict as pubmed_list
UNWIND pubmed_list as article
    MERGE (p:Publication {pmid: COALESCE (article.pmid, 'NO-PMID')})
    ON CREATE SET p.title = article.article_title, p.journal_title = article.journal_title, p.db = 'pubmed'

However, when I add this:

    MERGE (dc:Date { year: COALESCE (article.year_revised, 'NO-YEAR-COMPLETION') })
    MERGE (p)-[:COMPLETED]->(dc)

    MERGE (dr:Date { year: COALESCE (article.year_completed, 'NO-YEAR-REVISION')})
    MERGE (p)-[:REVISED]->(dr)

The dict I am passing for UNWINDing looks also perfectly fine:

EDIT 2 - CONSISTENCY CHECK

After running a consistency check:

$ "$NEO4J_HOME"/bin/neo4j-admin check-consistency --database=graph.db

It starts like this:

2018-07-20 08:41:15.337+0000 INFO [o.n.k.i.s.f.RecordFormatSelector] Selected RecordFormat:StandardV3_4[v0.A.9] record format from store /Users/rich/Library/Application Support/Neo4j Desktop/Application/neo4jDatabases/database-4ca696a4-add3-4f69-a97b-1b89ce8e854b/installation-3.4.0/data/databases/graph.db
2018-07-20 08:41:15.340+0000 INFO [o.n.k.i.s.f.RecordFormatSelector] Format not configured. Selected format from the store: RecordFormat:StandardV3_4[v0.A.9]

Then I get a few thousands of these:

2018-07-20 08:42:49.837+0000 ERROR [o.n.c.ConsistencyCheckService] The referenced relationship group record is not in use.
    Node[145800979,used=true,group=8661147,prop=275750014,labels=Inline(0x1000000000:[0]),light,secondaryUnitId=-1]
    Inconsistent with: RelationshipGroup[8661147,type=0,out=0,in=0,loop=0,prev=-1,next=0,used=false,owner=0,secondaryUnitId=-1]

Then it continues with:

...  10%
....................  20%
................2018-07-20 08:49:01.023+0000 ERROR [o.n.c.ConsistencyCheckService] This record should be the first in the source chain, but the source node does not reference this record.

Now it runs forever with the error described before, like this:

Inconsistent with: Node[6577082,used=true,rel=329016678,prop=11682171,labels=Inline(0x1000000000:[0]),light,secondaryUnitId=-1]
2018-07-20 09:07:05.171+0000 ERROR [o.n.c.ConsistencyCheckService] This record should be the first in the target chain, but the target node does not reference this record.
    Relationship[314188818,used=true,source=140353187,target=78456590,type=4,sPrev=314188819,sNext=314188812,tCount=32,tNext=301551374,prop=-1,secondaryUnitId=-1,!sFirst, tFirst]

and this:

2018-07-20 09:08:14.715+0000 ERROR [o.n.c.ConsistencyCheckService] The next record in the target chain does not have this record as its previous record.
    Relationship[192261,used=true,source=28336,target=5180,type=4,sPrev=192264,sNext=192260,tPrev=191008,tNext=184760,prop=-1,secondaryUnitId=-1,!sFirst,!tFirst]
    Inconsistent with: Relationship[184760,used=true,source=24761,target=5180,type=4,sPrev=184750,sNext=184761,tPrev=184738,tNext=192261,prop=-1,secondaryUnitId=-1,!sFirst,!tFirst]

What does it mean for my database and how could I fix it?

Neo4j - NOT PART OF CHAIN! RelationshipTraversalCursor

EDIT - TRYING TO DEBUG

EDIT 2 - CONSISTENCY CHECK