Did you find a solution? I sometimes get this error pop up when one of the sources files are in use. It's one of those unhelpful error messages that happens when nothing else is obviously an error.
Thanks for asking. No solution yet. I did manage to get four of the necessary six companies to complete through the flow, so that's a bit of an update from my original post. And the other companies run fine by themselves. Source files being in use is probably not a cause, since my data is in MySQL, which does not lock records from reads.
Would love for someone to have ideas on how to troubleshoot, or at least make sense of the Prep log files.