After pursuing this in this forum for over a month, posting a bug and having it promptly closed as "not a defect", and no working solution, I'm left to conclude that it is not possible to get rid of the "your reverse proxy set up is broken" with recent versions of Jenkins and IIS.
Or, nobody here uses IIS as a reverse proxy. Or, folks who use IIS as a reverse proxy ignore the error message. Terry -- You received this message because you are subscribed to the Google Groups "Jenkins Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to jenkinsci-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/jenkinsci-users/b838616b-e50d-422b-bc93-c700aed859dc%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
