I hear you Steve. I just went through a strange ride with no sound explanation but I always ride high and hard because of problems in the past. I will list my actions concerning this matter for future reference.
Cookie and History, etc. delete of all content: No resolution.
Close and reopen all browsers: No resolution.
Database restore to older safe date: No resolution.
Database restore to latest backup: No resolution.
Run system check: Mid way through system check I was able to log in and remain logged in at Community.
System scan has not reached area that would affect browser or Internet prior to log in success.
Noticed extreme difference in site load time (drag server side) and then all is well.
Problem appears to have been server side at this time. Will report back if able to re-create or if problem persists.
Ancient and Honourable, John West.