Problems with Android App

I selfhosted jitsi on a v-server for my daughters school. I installed Jitsi with Docker and a certificate from Lets Encrypte.
The connections with Google Chrome and the iOS app works fine. Only with the Android app it does not always work. Some parents can’t get a connection with the Android app. For some parents the connection works.

If I set the default server for an Android app it works. If I change the server to my address it does not work anymore. So it is up to my server.

The version of the Android app is 20.1.0 build 3883698. Does anyone have any idea what it might be?