Thank you for your post.
Unfortunately, there isn't enough information provided that points me to possible causes. You can always search the server logs with FileMaker Server 5.5 and see if anything catches your attention.
On a side note, the file format changed from FileMaker Pro 6 to FileMaker Pro 7, so FileMaker Pro 7 clients would be unable to use files hosted on FileMaker Server 5.5.
If you need help with the server logs, let me know.