Findfast file limit

Hi,
having illegal operation errors on site within Word. They use word as a means for creating invoices (?) and have nearly 20,000 documents divided between half a dozen folders. They HAVE to use findfast to allow them to search the dcocments. However, frequently while browsing they will get ‘illegal operation’ error messages. Not specific to a pc. What I am wondering is if there is a limit for either findfast or word to the number of files that can be browsed.