Hmm, it's working now. I tried running the install script again, using these paths:
http://domainname.com/ubbthreads
D:/Data/domainname.com/ubbthreads
Whereas before I had:
http://domainname.com//ubbthreads
D:\Data\domainname.com\ubbthreads
Maybe UBB.Threads doesn't support backslashes, or the extra slash in the domain was throwing it for a loop. I'd suggest to the design team to 'touchup' these addresses so they will change backslashes to forwardslashes and remove trailing slashes if necessary.
Thanks! Shane