And you have more than 255 layouts in your file? This is a known issue on Windows systems.
For More Information see: http://filemaker.custhelp.com/cgi-bin/filemaker.cfg/php/enduser/std_adp.php?p_faqid=548
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