Hi Gerrit,
Maybe this concerns the same kind of issue?
https://forum.mendix.com/link/questions/89025
It is suggested here that the problem has to do with autocommitted objects, but that the error you described is thrown because there is no user context (which, in this case, also seems valid since you are executing the microflow from the queue)
So perhaps a good pointer is to check whether the objects you are using in the microflow aren't committed properly and whether there is any mentioning of autocommitted objects in your log at all?