Sometimes the session-warning from the X3 diagnostics page could be mistaken, but in the case where you can't actually login, it's very likely that there is some flaw with your PHP sessions (maybe related to open_basedir, but not necessarily). When you LOGIN from the panel, X3 will simply use PHP session() function, and if the login is correct but the session storage is not working, it will simply return to the login page.
What exactly happens when you try to login? Do you see a warning "incorrect login" or does it just show the login page again with no warning? Do you have a link for me to check? I can only diagnose properly if I can check myself.
ulfklose wrote:/var/www/vhosts/hostingxxxxxx.asdf.netcup.net/httpdocs/domain.tld/:/tmp/:/var/lib/php5/sessions:/var/lib/php/sessions:/var/www/vhosts/hostingxxxxxx.asdf.netcup.net/tmp
It could be diagnostics is wrong, and it could be open_basedir has nothing to do with the issue. From my experience, it's normally related to PHP session() (unless you are missing a PHP extension, but you would see that in diagnostics). One issue I noticed a few servers have had, is that session() was not persistent (across pages) ... Which means that even after successful login, the next page does not remember the login, and instead redirects back to the login page, without any warnings.
ulfklose wrote:I'm on PHP 7.4 but also tried 7.1, 7.2 and 7.3.
It's defintely not related to PHP version specifically. Likely your host has similar options configured across multiple PHP versions.
ulfklose wrote:My hosting provider allows two different settings for the open_basedir variable, I tried both. Both contain /var/lib/php/sessions thus this shouldn't be a problem.
Yes, it might be entirely unrelated to open_basedir.
The ultimate check is to see if PHP sessions() are working properly (persistently) on your server, which I suspect is not the case. Any application with login mechansim will require PHP sessions to work correctly. I would probably need FTP login to accurately be able to verify PHP sessions.