A nagging problem we've seen every now and then with FAST is the big red message box "Search is Unavailable" when you go into FAST Home to look at collection details. But when you try to execute a search, it works fine. In fact, everything works fine - just really slowly.
In the past when we've seen this on development systems, we typically reinstall and muck with things until it works - or we give up and go to a different server. Knowing the answer is nice, but getting something running is usually more urgent.
Last week, I finally broke down and asked a brilliant FAST systems engineer in San Francisco about the error. He had the answer in seconds - as usual, if you know the answer, it's pretty simple: ESP was not able to resolve to the system name it thinks it's running on.
In our case, we had installed ESP on a server named MARS - but nslookup resolves that to MARS.INTRANET.NEWIDEAENGINEEING.NET; hence the error. Luckily, I was able to solve the problem without making big domain changes or such - simply adding that entry in my hosts file was sufficient to keep FAST happy; the error went away without even restarting the server.
Applies to Microsoft FAST ESP, Microsoft FAST FSIA, and Microsoft FAST FSIS.