@opera-user12 Big Sur 11.7.4 also has this issue. Confirmed on 3 different computers with Big Sur. 11.7.1 & 11.7.3.
Looking for reason this might be happening but I'm thinking it's to do with permissions not allowing data to be stored or edited. Reports mirror those with macs trying to import csv passwords without success.
So far have tried enable full disk access for Opera and changing sharing & perms on both com.operasoftware.OperaGX folders, cache and App Support, to chmod 777 everyone. No dice.
I'll see if a VM with pre-monterey works.