I did now finally have time to track down this issue ...
I checked again our production servers and there we indeed have the HTTPEnableConnectorHeaders=1 set. We distribute this via a configuration document and I have to find out why - we don't use IIS front end servers. I guess we should be able to disable it and then perhaps see what fails ... if anything. :-)
I also found out why I saw on my test server what I thought is the same problem: I made a typing error for the SSL keyring file so it was not loaded and I always overlooked the error message telling me so ... how embarrassing. So after fixing this the "Redirect to SSL" problem of course went away.
Sorry for wasting so much of your time!
I guess for the case of HTTPEnableConnectorHeaders=1 this will be fixed in an upcoming fixpack?