I ran into an interesting and bizarre issue today when working to set up a new UM server for the Exchange User Group presentation next week. Everything was working well, on an old IBM Thinkpad A30p, except when I tried accessing the Directory using OVA. UM would respond with a short (1 second or less) delay and then say "Sorry I can't help with that" and then hang up. OUCH! Nothing was originally in the event logs that defined the issue. No TAP info or Google query found anything like this. After reading the logs and CHM file again, I still found nothing. I played with changes to the UM settings, OAB settings, added a PF for OAB to rest in (just in case), nothing helped. I turned up event logging levels to medium, then maximum, still nothing.
Finally I turned up logging to level 7 on the different UM aspects and got two event log warnings stating that GAL.CFG and DistributionList.CFG couldn't be found in the path C:\Program files\Microsoft\Exchange Server\UnifiedMessaging\grammar\en. I looked on the install CD and there the files were. I copied the files into the directory from the CD image and the Directory access worked immediately. This was a 32 bit RTM build, so maybe this only happens in 32 bit and not 64 bit.
I will have to try a clean install with SP1 and see if this happens again.