Not sure about mac. But if you are using Windows you cant delete files while the server is running. Try replacing the old one for this http://palmergames.com/downloads/chatdev/TownyChat.jar.
Nice, no more error after i update towny but when the server autosaved i see new error: http://pastebin.com/yz0W5Fxk Can i solve it? Thanks p.s. i use CentOS, if I stop restart my server can i solve my errors? EDIT by Moderator: merged posts, please use the edit button instead of double posting.
Velerium, sounds like you've got a bogus session.lock file floating around in one of your world folders. Try shutting down the server, removing all of the session.lock files from your world folders, and starting it up again.