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The HTTP 501 is the error in the HTTP POST handling in the servlet engine of Domino HTTP. There is a hot fix for that, but you need to open a PMR to get that hot fix. The issue was that HTTP was not reading the body of the POST AND not closing the connection for the auth error, so the bytes were still there when the next request came in which gives you the error like you reported here.
Companion 2.0.5 changed the authentication to improve some areas (obviously broke some others like this HTTP issue), so that is why we saw it now but hadn't before. Also, you have to be using the form login and not HTTP 401 login to hit it (401 being the recommended method), so not everybody will see it now either.
Stefan opened the first PMR which ended up with that HTTP hot fix, so Stefan should already have that HTTP fix. Once you get the HTTP fix, if you still have problems, I need a PMR for the HTTP and Traveler logs too.
Feedback response number RSSN8SLG67 created by Robert S Sielken on 03/21/2012