Uncheck the "Keep me logged in" box during login. This is a bug with Internet Explorer since many version of UBB.threads.
https://www.ubbcentral.com/forums/u...remember-me-problem-in-internet-explorer
https://www.ubbcentral.com/forums/ubbthreads.php/topics/243975/member-login-issues
just to list two (of a several hundred)

If you do a search for "Internet Explorer" on the UBB.threads support forums, you'll see that the posts go way back to even v6 in 2005.

I believe Microsoft Edge finally has the cookie issues fixed.

Current developer of UBB.threads PHP Forum Software
Current Release: UBBT 7.7.5 // Preview: UBBT 8.0.0
isaac @ id242.com // my forum @ CelicaHobby.com