Android version: 7.1.2
Device model: Asus Transformer Pad (TF701T)
Stock or customized system: KatKiss ROM - Android 7.1
Nextcloud app version: 3.13.1
Nextcloud server version: 19.0.3
Thanks for your help!
Can you provide us a screenshot?
I do not fully understand the issue.
Sure, here's the mentioned website layout that I get:

Furthermore I was able to reproduce this behaviour on another tablet coming with the manufacturer's Android (7.0).
I found out that it must have something to do with the enabled Two-Factor TOTP Provider. I was able to login to another Nextcloud instance that is not using TOTP and see the correct Android app layout:

Unfortunately, disabling TOTP in the user security settings leads to a hanging login page (On PC the login in the Firefox browser works, in Brave only after refreshing the page):

(For privacy reasons I removed my Nextcloud logo and parts of my account name.)
Hi again, I was able to solve the issue by myself. It was related to a server misconfiguration.
After adding 'overwritehost' => 'XXXX.de', 'overwriteprotocol' => 'https', to config.php as mentioned here, the hanging login problem with Brave was gone, but also the wrong layout with TOTP enabled disappeared!
Most helpful comment
Hi again, I was able to solve the issue by myself. It was related to a server misconfiguration.
After adding
'overwritehost' => 'XXXX.de', 'overwriteprotocol' => 'https',to config.php as mentioned here, the hanging login problem with Brave was gone, but also the wrong layout with TOTP enabled disappeared!