There was a problem with some special characters (they were not included in validation), and password manager would generate a password that is pretty good, but doesn’t have special characters I’ve initially chosen.
Now, I’m including most special characters, so it should work.
PS: Also, the pre-flight validation now includes special character check too, so you can see the validity real-time instead of after you submit. (This check was not originally included on the client-side, only on the server).