Just a few little gotcha's I run into recently when using the HTTP Adapter:
Problem: (405) Method Not Allowed
Resolution: Allow Scripts and Executables to run in the folder BTSHTTPReceive.dll is sitting in
Problem: (500) Internal Server Error
Resolution: Make sure the identity the application pool for the web site runs under has access to the BiztalkMsgBoxDb. You can do this by adding the account to the Biztalk Isolated Host Users Group.