What is your chosen downloader?
It can be a number of things, it could be something in the header being sent to the "mini" server for FG is not done properly?
It can be that something being passed to the downloader is not parsed correctly?
If you wouldn't mind, can you provide the link where this happens, and what downloader you are using and which which version of FG.
Have you tried the
FG Latest Dev Build?
Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/36.0.1941.0 Safari/537.36