Sharing from Smartphone to Nextcloud not possible

When I send files to a Nextcloud directory using the Share feature, I get the following error message:
Upload failed
The operation cannot be completed, the server is not available

For example, I had tried it with a photo app and from the “ES File Explorer”. But the copy and paste works as expected.
With my tablet (also Android) it works as expected. So I suspect that some setting of the A3 is to blame. But I cannot imagine which one.

Maybe someone has had this problem before and can help me.

Nextcloud version: 17.0.2
Operating system and version: Windows 10 64bit
Smartphone: Samsung Galaxy A3 (2017), Android 8.0.0