Bad Request error on form submit

I created a plugin which collects data from a form and submits it via ssl / fsock to another server. The plugin works correctly and all the data gets submitted correctly to the target and the server log from the target also says that everything is ok. However when I echo the result locally it gives me a "BAD REQUEST Your browser sent a request that this server could not understand" - message. As mentioned, everything works fine and sure I can ignore the error but I would love to get rid of it. Any ideas?

This is kinda hard to debug without the exact content of the request. Could really be anything. More info please :)