The issue I have ran across is that Internet Explorer (which is used by FMP on Windows) limits the length of URLs to 2,083 characters and Safari allow up 80,000 character. I had problem with FMEasySignature, which can be found at www.fmeasysignature.com. I modified the program to use the clipboard. Here is a sample of my modification
User name and password for my modified database is supervisor and password.
This may or may not be related to your issue, but hopefully it will help.