You have blocked the Google Hangouts application using a Content Filtering (CF) policy.
As a result, your users are unable to join video calls using Google Meet (meet.google.com).
HAR file shows HTTP Error 403 - Forbidden: Access is denied.
Request URL: https://meet.google.com/hangouts/v1_meetings/media_sessions/callperf?key=AIzaSyCG_6Rm6c7ucLr2NwAq33-vluCp2VfSkf0&alt=proto
Request Method: POST
Status Code: 403
Remote Address: 188.8.131.52:443
Referrer Policy: origin-when-cross-origin
In the above request URL, the path "google.com/hangouts" is matching the Hangouts web app in Content Filtering.
Although meet.google.com is not part of the application, this is enough for CF controls to trigger.
Update the CF policy for blocking Google Hangouts from using a web app-based condition to URL-based condition.
Block the specific URL hangouts.google.com.
Google Documentation: IPs to whitelist for Hangouts/Meet