The problem can be with the size of a letter. I tried to send one message multiple times and each time got 500 error. When I shortened that message it passed. I tried to change it after that adding what was not got in, but it gives 500 error again. So only short enough messages can be posted or sent.
And we probably will have a problem with multiple sent messages as many don't see that a message is sent when it was. Guys please check the topic you send a message after clocking the post button if nothing happens. It is possible that your message is already there even if nothing happened.