I can provide more details if needed but...
In xpra/net/websockets/common.py client_upgrade()
The request header for "Host" should be the server host not client host. This is causing issues when using it through a reverse proxy which is proxying based on this host header.
Tested on r21875
Should be fixed in r21887.
@mjharkin: does that work for you?
Yes, r21887 works for me.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2175