Since i was browsing code i came up with the thought - you probably trying to put an item not on primary cell, like if you tried to put in smaller cells of mechanism or into theta slots, code didn't age well - SVM only supports primary, will be fixed in SPT 4.0 however.
I think I found out why, it's backpack-specific, apparently.
I tried to insert a THICC Item Case into a RUSH 100, and while it is empty and more than capable of holding a THICC in it, it will say item is not compatible. Even the smaller medicine case is rejected.
6Sh118 raid backpack will fit most cases just fine.
EDIT: I think the new backpacks added for 0.16 has this problem.