WordPress Admin Login Error Fix

I run the site bizasgroup.gr and suddenly the admin dashboard throws an error message the moment I try to log in. Because of this I’m locked out and can’t manage content or plug-ins. Everything on the public-facing side still works, so the problem is isolated to the back-end authentication flow.

I need someone to trace the source of the error (theme, plug-in conflict, corrupted core file, database issue, etc.), repair it, and confirm that I can log in normally again without triggering new warnings. A short explanation of what was wrong and how you fixed it will help me avoid the same situation in the future.

Deliverable
• Restored access to wp-admin on bizasgroup.gr, with the error fully resolved and no new issues introduced.