Login problems maybe related to PHP version

I have a site with login problems.
Using sitename/wp-login.php gives me an error message > critical error
Using sitename/wp-admin.php works better, I get logged in, but there is still an error message about a missing page, but not which missing page…

I got an advice to go back to PHP 7.4 but that dis not solve the problem.
So Im having the same problem in 7.4 and 8.0

Hi there

Im so sorry but such a problem can not be a theme issue, start it on wordpress.org or similar free forums