Corda flow exception: net.corda.core.flows.NotaryFlow$Client is not registered

Scenario: running 2 Corda nodes (say, A and B) that are running within a larger zone. There is 1 notary node in the zone.

I have 2 different flows: the first flow f1 only runs on node A and only creates transactions that are stored locally (as in, no other nodes are involved). The second flow f2 also only runs on node A and creates a transaction that modifies the states inserted by flow f1 (s1, s2, ...) by adding an input-output pair to the transaction where the input states are the existing (s1, s2, ...) states and the output states are the modified states (s1', s2', ...). Flow f2 also creates an additional output state, therefore we have Sn input states and S(n+1) output states. The transaction created in flow f2 is shared with node B.

Flow f1 runs fine, but as soon as I run flow f2 I receive the following error in Node A:

net.corda.core.flows.UnexpectedFlowEndException: net.corda.core.flows.NotaryFlow$Client is not registered. In the logs of the notary I can see the same error, and when I check the logs for nodeB it says: net.corda.core.flows.UnexpectedFlowEndException: Counter-flow errored

I have local tests, using a mock network that is able to run fine. I currently have no clue on how to debug this issue or whether it is a mistake in the flow itself, or something else. I am running version 4.5.8

Below is the code snippet for creating the transaction in flow f2 (variables have been renamed):

val txBuilder = TransactionBuilder(notary)
            .addOutputState(additionalState, CustomContract.ID)
            .addCommand(
                Command(
                    contract,
                    additionalState.participants.map { it.owningKey })
            )
        existingStates.forEach { txBuilder.addInputState(serviceHub.toStateAndRef<CustomState>(it.ref)) }
        existingStatesModified.forEach { txBuilder.addOutputState(it, CustomContract.ID) }





        // Stage 2.
        progressTracker.currentStep = VERIFYING_TRANSACTION
        // Verify that the transaction is valid.
        txBuilder.verify(serviceHub)

        // Stage 3.
        progressTracker.currentStep = SIGNING_TRANSACTION
        // Sign the transaction.
        val partSignedTx = serviceHub.signInitialTransaction(txBuilder)

        // Stage 4.
        progressTracker.currentStep = GATHERING_SIGS
        // Send the state to the counterparty, and receive it back with their signature.
        val otherPartySessions = counterParties.map { initiateFlow(it!!) }
        val fullySignedTx =
            subFlow(CollectSignaturesFlow(partSignedTx, otherPartySessions, GATHERING_SIGS.childProgressTracker()))

        // Stage 5.
        progressTracker.currentStep = FINALISING_TRANSACTION
        // Notarise and record the transaction in both parties' vaults.
        require(insertEvent(insuranceEventState.fullEvent)) { "Unable to insert event data into the triple store." }
        return subFlow(FinalityFlow(fullySignedTx, otherPartySessions, FINALISING_TRANSACTION.childProgressTracker()))```

Turns out this was caused by an incorrectly configured notary (which was out of my control).