#DATASYNC not disappearing after fixing merge

I have a report with 4 queries. I used two of them in my report. I merged two of them in a report and the report runs fine. However, if I change anything in the merge (eg. add another dimension to the merge or unmerge) the report only shows #Datasync.

If I repair the merge as it was, the report still shows #Datasync. How is that possible?

So to clarify:

1. Report works.
2. Change made in the merge: report fails with #DATASYNC
3. Merge restored as it was: report still fails with #DATASYNC. Report needs to be completely rebuilt.

Objects on different levels were combined (user tried to show email of companyemployee next to companyname). Logic dictates that this does not work.