This is a problem with the SSL cert generated by the new Filemaker Pro 8 install. The "solution" as found http://bit.ly/5VvRe is to:
# Deactivate FileMaker Pro if applicable (select Help > Deactivate) and un-install FileMaker Pro.
# Set the client machine system clock to a time prior to September 22, 2008.
# Re-install FileMaker Pro.
# Set the client machine system clock to the current time.
Or - you can copy the server.pem cert from another client and put it in Applications -> FMP -> (right click) -> show package -> /Contents/MacOS -> server.pem (not recommended, as you really don't want multiple copies of a cert floating around).
This issue is fixed by updated versions of FMP 9.