Hi.   We just had this happen to us with a Hudson/Jenkins type of RTC build engine.  We had changed the Jenkins server in an existing build engine to refer to a new Jenkins machine //jenkins2 instead of //jenkins1.  I had updated the RTC Build Engine to change the https address of the Jenkins server, and tested the connection (and it was fine, and done with authentication), and the connection itself worked.   Builds instated from Jenkins worked fine.   But, if we did a right-click in the RTC client of a build definition that used this now updated build engine pointing to the now Jenkins server, we would get the error reported here.    If we expanded Build options in the build definition, and manually selected the build engine (even though it was the only build engine named in the build definition), then it would work without this error.

To get around this, we ended up having to create a new  build engine pointing to the to the same Jenkins server, and then manually going through 50 or 60 build definitions, and switching the supporting build engine to the new one, and removing the old one.  The the Request Build would work without his error.

There are three things you need to make sure are set for your engine to have it process requests.  I am assuming this is a Jazz Build Engine, but you need to make sure that the engine is active, it is supporting the right build definitions, and it has request processing enabled (if JBE).