Android Client version > 3.13.1 not connecting to my Nextcloud

Hi guys,
i need your help.
It is not possible to connect any Android Client > 3.13.1 to my Nextcloud.
If you try to connect, it is saying “Testing connection”, than after a minute its saying:
The server takes to long to respond.
This is happening on all Android Devices i have tested, 3 phones and one tablet (Android 11 and 10), with WLAN and Mobile Internet.
When i downgrade to a version of the android client <= 3.13.1 all is working again.
I cant see anything related to that in my Server log, not in the nextcloud log ant not in the access log.
Can anybody help me with that?
Thanks and best regards
Benedikt Hilbert