I have 2 different websites running SilverStripe on a VPS (one running SS4.7.2, the other SS3.6). Both have been working well for several years, but the SS3.6 installation suddenly stopped allowing logins to the CMS (no changes were made to files on the server and when I download a backup of the database to test on my local machine, login works fine and gives appropriate error messages for incorrect entries).
The problem: SS3.6 installation on server will simply refresh the login page (without logging in) when any user tries to login - no error messages are given for incorrect entries, same behaviour on different browsers/devices and same problem if reset password option is used (with no email sent). The last successful login occurred 3 days ago and also correctly triggered a password reset link. I have tried rolling back to an earlier backup of files on the server, but am still getting the same problem. I cannot run dev/build as it requires me to login and even admin logins just refresh the page.
I suspect that a server config file or SS log files may be causing the issue, but have no idea where to start looking or how to test for this to pinpoint the issue. Any tips on debugging this and fixing it would be greatly appreciated!