Its seems on Mono log is receiving from nginx:

On error:
[2010-03-15 11:27:09Z] Debug Accepting an incoming connection.
[2010-03-15 11:27:09Z] Debug Record received. (Type: Params, ID: 1, Length: 529)
[2010-03-15 11:27:09Z] Debug Record received. (Type: BeginRequest, ID: 1, Length: 8)
[2010-03-15 11:27:09Z] Debug Record received. (Type: Params, ID: 1, Length: 0)

On sucess:
[2010-03-15 11:27:09Z] Debug Accepting an incoming connection.
[2010-03-15 11:27:09Z] Debug Record received. (Type: BeginRequest, ID: 1, Length: 8)
[2010-03-15 11:27:09Z] Debug Record received. (Type: Params, ID: 1, Length: 529)
[2010-03-15 11:27:09Z] Debug Record received. (Type: Params, ID: 1, Length: 0)

It seems when it gives this error, Nginx injected first the Params instead of the BeginRequest.