Websocket request origin not allowed by Upgrader.CheckOrigin reverse proxy with Collabora Online Development Edition

I have recently upgraded mattermost server. IT is now version

Mattermost Version: 6.7.0
Database Schema Version: 82
Database: mysql

just before this version I had installed on the same virtual machine CODE (Collabora Online Development Edition)
I had this working with Mattermost plugin Collabora Online. My domain is web.domain.ca, however the Collabora Online on the same machine points to docs.domain.ca with reverse proxy

Mattermost config/config.json has "AllowCorsFrom": "*" yet if I refresh the client I get a flash of the UI then a blank and mattermost log shows

{"timestamp":"2022-03-23 15:33:29.841 -04:00","level":"error","msg":"Failed to upgrade websocket connection.","caller":"web/context.go:105","path":"/api/v4/websocket","request_id":"gt5iahrbn3b38yfozptq17p9sy","ip_addr":"x.x.x.x","user_id":"biixsuayf7f4mxo1w5bi3xei1o","method":"GET","err_where":"connect","http_code":500,"err_details":"websocket: request origin not allowed by Upgrader.CheckOrigin"}

This was working fine before. Has anyone run into this problem?