are you exporting to .xls or .xlsx? if .xls, try .xlsx--the newer excel format.
Also, if you haven't already, try creating a small database from scratch with just a few fields and one or two records. Can you export from it?
Greetings Len Weston,
Thank you for posting.
From Answer ID: 11007 http://help.filemaker.com/app/answers/detail/a_id/11007/kw/Lion%20Compatibility
I would suggest you download a 30 free trial of FileMaker 12 and see if the problem persists. Also, as PhilModJunk alluded to OS X 10.7 and newer does not support legacy excel formats. It could be that you are attempting to export to a format that is not supported by the operating system itself.