weird weird weird
The submission where the error happened was id 2545 in journal 30
I saw this issue Upgrade from OJS 2.4.7 to 3.0.1 fails - #18 by asmecher and used the sql you have there. I found out that there were submissions left in the system from two journals that have been removed some time ago (journals 45 and 67) and these had left behind some submissions which I removed.
That was all I did, but after that the upgrade has now passed the moveReviewerFiles function.
I am clueless what happened, any idea @asmecher?
edit: Also, does removing a journal leave the items in submissions table intact at the moment?