I may have found the issue. But first...
I checked in Keychain and all certificates are set to "always trust". (I have no idea how to figure out which one of dozens of certificates is actually for itch.app, short of opening each one. Can you specify a certificate name?)
I then opened Terminal and typed in...
spctl -a -vvvv /Applications/itch.app
...which returned...
/Applications/itch.app: cannot find code object on disk
Upon searching for itch.app, I discovered that the itch installation program put it in ~/Applications, which has never happened before. It's always been put in /Applications. I tried again, typing the same command into Terminal with the correct directory, which returned...
/Users/NikkiNyx/Applications/itch.app: accepted source=Developer ID origin=Developer ID Application: Amos Wenger (B2N6FSRTPV)
...so it seems the certificate is valid. Yet I still get the same x509 error message on login.
I'm wondering if the installation is the problem. Why is itch.app suddenly being put into ~/Applications instead of /Applications? And is that affecting how the app accesses what it needs, like a valid certificate? (Note: Moving it didn't help, unsurprisingly. Still couldn't login.)
Except for you, I feel like itch really doesn't give a crap that I can't access games I've paid for. This issue has been going on for several months now, and it's one of the many reasons I bloody hate game website apps. Half the time, they don't work. Plus, they take up a crapload of space to basically function as a folder and connection to the website (mining my gaming habits in the bargain). I have four different game website apps and they all suck.
Sorry, but I'm seriously pissed that I can't play games I've paid for. I do appreciate all the work you've done. Please let me know what I should try next.