by Ken Task.
So am trying to understand ... you upgraded X number of Moodle 2.6.x sites via a mini-march to 3.5.1.
Then ran autobackups on courses. And now you are trying to restore those autobackups to another 3.5.1 in order to 'merge' all sites into one.
So did the upgrades to the 2.6's go off without any errors? Got them to 3.5.1 with no issues?
Did you check any of the courses for course formats addons after those 2.6's were mini-marched to 3.5.1's?
Do you have any addons for course formats that are need of updating? Are the course formats that were in the 2.6's compat with 3.5.1?
Have to confess, have never seen that error before. Above is really guessing.
'spirit of sharing', Ken