Re-enable the Assignment 2.2 (which is the old assignment tool called assignment), then try it again. The new assignment is (behind the scenes) now called 'assign', I think. After conversion, then hide Assignment 2.2 again so that teachers will not have an option to select it when creating a new resource.
If importing older version backups, you might have to turn it on again as the restore process may not convert on the fly.
Might also be time to consider that older backups might have other issues as well.
'spirit of sharing', Ken