Unable to log into server and log out of server

Sounds unusual, yes. When I try to join a server I most likely get an Internal Server Error message. On the odd chance that I am able to get in, I am in the game for about 30 seconds with extreme lag. I then get the Internal Server Error message. My server has a website that shows who is currently on the server and I am listed as being online after I join, but when I am kicked, it does not log me out. I then am unable to try join again at all until the server admin manually kicks me from the server.
Edit: Apparently whenever have have tried to join the server, it has made the server extremely laggy for other players. My most recent attempt actually crashed the server.