@EmreTheLizard I tried the procedure you outlined, but it failed: I added one site to Windows Key Manager (and it recorded it well: when I closed it and reopened, it was listed there - but with a mention of "LegacyGeneric", no idea what this means), but Opera ignored it, even after having restarted it: that web site does prompt for the correct login, but no password. To make clear, it is configured to store the passwords I enter. Even before this total erasure, there were sites Opera just stopped recording the passwords (while it had in the past). Now it just doesn't show any past recorded password nor record new ones.
Also, I used NirSoft's WebBrowserPassView in order to display the stored passwords for all my installed browsers: for opera, it displayed a list of sites, their respective logins/accounjt names, but empty passwords (even after having added one in Windows Key Manager).
I tried other web sites: for some, Opera stores the password now, for some it does not (while it did in the past). This is maddening. I may drop Opera and move to Microsoft Edge: with this test site (the one I used above), it offers to record the password, it stores it, and prompts it when I exit Edge and reenter.