Administrator unable to access admin area

[WIMorrison]

I have something very strange happening with my v2.1.4 forum as I am unable to access the admin area, being constantly asked to re-enter a password when I try to enter, despite being shown as logged in as admin. Sometimes after being asked to reenter the password I am presented with

"Session verification failed. Please try logging out and back in again, and then try again."

but sometimes I get

"Your session timed out while posting. Please go back and try again."

Additionally when some of the users login they are shown as being logged in another user, I have also seen this with the admin account.

I have switched off SMF Caching to remove this from the equation, and am at a total loss as to what on earth could be wrong. This started yesterday and it has me totally foxed. I am also going to place the forum in Maintenance Mode by editing settings.php to prevent anyone writing anything whilst I try to resolve this.

Any suggestions what could be causing this?

[Doug Heffernan]

This started yesterday and it has me totally foxed.

Were there any changes/modifications done to either the forum or the server prior to that happening? I'm asking because issues don't happen out of nowhere. Something triggers them.

[Aleksi "Lex" Kilpinen]

Additionally when some of the users login they are shown as being logged in another user, I have also seen this with the admin account.

This is a telltale sign of a server side cache not properly configured for dynamic content.
You should probably ask your host about this.

[Kindred]

if your host uses Varnish - ask them to turn it off