This post is for informational purposes only.
I maintain a fresh install of Windows XP SP3 w/IE8 only for testing and troubleshooting.
This is not my main setup, so the VM described below is purely for testing feature scenarios on a fresh system without any other installations or custom settings interrupting the test environment.
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Here is what I have:
Fresh install of Windows XP 2002.
Installed all the windows updates and updated to Service Pack 3.
Updated to the most recent version of Internet Explorer for Windows XP, IE8.

Here is what I did:
1. Using IE8, I browsed to the login page:
https://www.ubbcentral.com/forums/ubbthreads.php/ubb/login

2. Entered my credentials.

3. I was then sent back to the main page (ubbthreads.php/forum_summary) where I was told, "You are not logged in. [Log In]"

4. I browsed to https://www.ubbcentral.com/forums/ubbthreads.php/ubb/mycookies.html to clear cookies, but the problem still persists.

This is a completely fresh install of "Windows XP Version 2002 Service Pack 3" with all Windows Updates installed. Nothing else is installed.

I don't have this login problem on any of my forums; It's only on ubbcentral.com.

I also did a completely fresh install of UBBThreads 7.5.8 to a sandboxed server. I could log in and navigate as expected, using IE8.

To further my testing on this fresh install of Windows XP, I installed Firefox v26.0; And could navigate to ubbcentral.com and log in successfully. No problems.

Summery:
My servers are running php5.2.17 and I have no problems logging in to a fresh or current install of UBBT v7.5.8 using either IE8 or FF26 on XP SP3. But, attempting to log in to UBBCentral.com with IE8, and only IE8, fails; FF26 is successful.


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