Our BorderManager points to an upstream proxy, and has done for the last
6 or 7 years. Recently https (secure)requests stopped getting through. I
believe I haven't changed anything on the BorderManager. The ISP claims
no changes to their proxy.

BorderManager returns the error page for '504 Gateway Time-Out' but this
is not completed, it stops after the description and ends with an open
HTML tag <font face =

If I bypass BorderManager with a browser and point directly to the
upstream proxy then https goes through OK

Any ideas what could cause this?