I have completed a trade with another user by mail and now whenever i click the "i have received my cards" button I am brought to an error message:

"500 Internal Server Error

If you are the administrator of this website, then please read this web application's log file to find out what went wrong."

Sorry for the problems, the server was improperly rebooted and there were email sending problems that affected site features such as trading and signups :(. Now it should be fine.