Have same issue, Logs from %localappdata%\Overwolf\Log\Apps\FTB App\index.html.log is just spamming this constantly (no logs in between, just this message) -
2020-12-24 13:06:58,255 (ERROR) </dist/desktop/js/chunk-vendors.4a66187c.js> (:227) - WebSocket connection to 'ws://localhost:13377/'...