check the screeny attached.
Your screenshot is of a "Paths & Database" setting.
Does this match the same URL in your general setting for "Homepage URL" ?
Do you also have your sessions path setup correctly?
as a side note, which I believe to be completely unrelated to the Windows paths issue you are currently experiencing:
The CSS validator you use, is it validating against CSS2.1 or CSS3?
You will get errors if you are validating CSS3 against a CSS2.1 file.
Is the file you are validating against something included with UBB.threads, or something you created or found online somewhere, such as a prior version of UBB.threads?
What are you validating? a url or a hostedhtml page, or direct link/copy-paste of a css file. a a backedup or exported ubbt style?
