I recently upgraded to moodle 3.6.3
All worked beautifully well for several days, and then out of nowhere cookie session issues appeared without me doing anything on the server (I'm the only user right now) - "Your session has timed out. Please login again" - and now I as admin cannot login anymore - at all - and my test user can login if persistent but still get the error "invalidsesskey - Your session has most likely timed out. Please log in again."
I have spent several hours exploring the many threads in the moodle forum that have been talking about this issue over the past decade, and tried everything suggested in there: Clearing cache, folder permissions 0777, repairing database, adding code to config.php, re-uploading a clean copy of moodle, confirmed that php.ini has session.auto_start = 0... but without luck.
I have seen https://docs.moodle.org/36/en/Session_handling but don't know if it applies.
Any suggestion how to troubleshoot this?