Manager Login problem

I just upgrade my site to 2.3.3. Site front enda appears fine. With manager I have problem. When I try to login I get "500 Internal Server Error". If I refresh the page Manager Control Panel appears (with delay, after 6-7 seconds)!!! I can log out fine, but when I try to login again the same thing happens. "500 Internal Server Error" and when I reload the page manager control panel appears (with delay again).

I have tried to delete cache from manager or server directly, flush permissions, create new accounts etc.

This is huge problem since my client cannot accept this.

Any ideas?

Have you cleared your browser cache and cookies?