I am in total distress, as I could not find the solution to this error.
My flow worked perfectly, until these days. It started having this problem yesterday and I can't get it to run without error unless it's connected to a gateway. I will not be able to use the gateway I tested, because the VM will not always be on.
The error happens in the step where a merge is made between two data sources: AZURE SQL SERVER and SHAREPOINT.
Have you tried using landing dataflows as I suggested? This can be done in a Pro workspace.
In the child dataflow:
1. Uncheck "Load" for the query fetching the landing dataflows ("Landing1" and "Landing2")
2. Create a new query by choosing "Reference" from the right click menu of the "Landing1" entity.
3. In that new query you can now Merge with Landing2. Because you're not loading the source dataflows, you're not using Linked Entities and you don't need Premium.
Try loading the SQL and Sharepoint in landing dataflows where you do nothing else, then merge these two dataflows in a third dataflow.