I realize that the versions for FileMaker and Mac OS don't match exactly, but see if this report matches yours. If if does, there's a fix described in the thread that you might be able to use to fix this issue.
For More Information see: http://forums.filemaker.com/posts/bc50c9fef9
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Thanks PhilModJunk! I tried the sugestions in the posts you mention but so far nothing works. I'll double check for Web Sharing but I doubt the Mac has it turned on. Maybe FMP10 does though.