/wp-admin does not log in, but wp-login.php does

I just moved to WordPress. WordPress files are in the subdirectory /m/, but I used htaccess to show them in the / root domain directory.

Everything is working, all css, plugins, etc. Everything is running smooth, except wp-admin login.

Both the home directory and WordPress directory are set to mydomain.com.

If I go to mydomain.com/wp-admin and it redirects to https://www.mydomain.com/wp-login.php?redirect_to=http%3A%2F%2Fwww.mydomain.com%2Fm%2Fwp-admin%2F&reauth=1

then login doesn’t do anything. There is no error message, it just refreshes the page and I enter my details again.

If I go to mydomain.com/wp-login.php directly, login usually works.

After about 60 seconds of being logged in, I get a popup saying: Session expired. Please log in again. You will not move away from this page.

I disabled my plugins already, so it’s not related to that.