@theopenem_admin - Don't chase this further - I believe this is a remnant from my failed attempt at upgrading MariaDB - https://forum.theopenem.com/topic/466/unable-to-to-login-system-invalidcastexception-object-cannot-be-cast-from-dbnull-to-other-types .
Back then I had to downgrade and because of all the database enconding and collation changes I had to manually edit my MariaDB 10.11 backup so I can have it restored under 10.5. I belive this is now causing my database schema, although working to be out of sync with the model/schema in your coude - hense 'The model backing the context has changed since the database was created.'. What I did was go back to a pre-MariaDB upgrade backup, had that restored, it properly updated to 1.5.5. and now I can login fine. Thanks for your help anyway!