Did you recently upgrade FileMaker 11 to FileMaker 11v3? There are reported issues with Entourage 2004 and FileMaker 11.v3.
For More Information see: email script doesn't work (entourage )
This is one of many acknowledged bugs that can be found in the Known Bug List here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html