Workaround
I added a new one manually, and recreated it.
Created bug report
https://bugs.kde.org/show_bug.cgi?id=490275
I added my Nextcloud account in KDE Online Account manager.
When I clicked the share, I got another sign in prompt, but this time is native to Dolphin. I entered the wrong password as I was supposed to use the app password.
Now, I’m getting a “does not exist” error.
Is there a way to force the sign out?
I think this should not block people from signing in if they got the password wrong.