We’re looking into the cause of the 500 errors that are being seen by users on twitter.com. We’re also working on the posting problems that many people are running into when trying to create tweets.
Update (Tuesday 3:43pm) We’re still looking into tweet creation problems. Specifically, reports of erroneous error messages and/or no error message, but inability to post tweets. Please add comments to the Known Issues page here if you’re experiencing this problem. Thanks for your patience.