@Meghana-MSFT @Prithvi-MSFT thank you for your responses.
We could also reproduce the problem on Android devices. The meeting Tab wouldn't load.
Turns out the problem was - missing intermediate certificates in the chain on our web server. Strange that some MS Teams clients (Win10, MacOS Catalina, iOS 15) can handle this, but others (MacOS Big Sur, Android 11) cannot.
Anyway, fixing the certificate file configured on NginX fixed the problem on all the clients.