Hi,
had the same issue. As the stacktrace reveals, it is a problem with the Websocket (handleWSUpgrade).
Fixed for me in Apache (>2.4.47). with adding this part to my reverse proxy config:
ProxyPass "/strapi/" "http://localhost:8081/" upgrade=websocket
(See more at: mod_proxy_http - Apache HTTP Server Version 2.4 )
Nginx probably similar issue, you need:
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
Best,
Martin