Android App - Server takes too long

Sorry for my bad english, i am a german guy.

Nextcloud version (eg, 12.0.2): Nextcloud 12.0.4
Operating system and version (eg, Ubuntu 17.04): Ubuntu 16.04 LTS
PHP version (eg, 7.1): php 7.0

The issue you are facing:

The Nextcloud app does not work on a specific smartphone. Server responds too slowly

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

The error in the app that the server takes too long to respond occurred without modification on the smartphone or server.
after I could not fix this with various ways, I have completely reinstalled the server on the same machine on a different hard drive. so the installation is completly clean.
i’ve tested the new server on a laptop and an other smartphone and it works without any errors, from the outside and the inside.

but the smartphone, which had the error, now brings this error again, to the new and absolutly clean server.

I have already uninstalled the app several times on the smartphone and reinstalled it again and again the error comes.

apparently reinstalling the entire server was useless and it’s up to something else. if i open on the smartphone the cloud through the browser it works without problems. Therefore, I think that it can only be due to the app.

The smartphone is a google pixel with android on it.

Do you have any ideas to fix it?

thank you for your help guys