I tried to migrate to Carbon, but only Box Looters (one of my most essential plugins) has problems invoking saves. I've attached a screenshot.
I would be very grateful if you could adapt Box Looters to Carbon.

Not having this issue with Carbon myself:

[BoxLooters] Saved Boxlooters data

Saved 128,590 ents, cache(0.27), write(0.05), disk(0.03).

Saving complete

Did you have any data? i.e. had you tried looting a box before the save?