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Dr. Thomas MillerDr. Thomas Miller 

ALL_OR_NONE_OPERATION_ROLLED_BACK when uploading contacts with Dataloader (Process Builder)

We are uploading test contacts (all for the same account) with Dataloader. There is a process builder flow which updates a count on account whenever a contact is added.
The upload runs successfully when 10 contacts are uploaded - but fails with ALL_OR_NONE_OPERATION_ROLLED_BACK when 20 contacts are uploaded. As all contacts are copies of the same the issue cannot be on the contact.
Looking into debug logs it seems that the flow engine defers the account record types to the very end (which makes sense) but then fails without any clear reason. Detailed error message is
"FLOW_ELEMENT_ERROR|The flow tried to update these records: 001D000000yg0j4IAA. This error occurred: ALL_OR_NONE_OPERATION_ROLLED_BACK: . You can look up ExceptionCode values in the <a href='https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_calls_concepts_core_data_objects.htm#'>SOAP API Developer Guide</a>."

Is there any undocumented restriction on the maximum of processes accessing the same object at the same time?