Android: Download failed for files except for Music and Photos

Please help us by providing the following info.

Expected behaviour
Donwload of Files (i.e. PDF)

Actual behaviour
Download failed for every file, that is not an image or music file (mp3, m4a)
- either klick for manual download
- or marking a folder for syncing

it tries to begin with download, but nothing is downloaded

Folders, music files and photos are both downloaded and synced without any problems
(iOS - no problem: owncloud on iPhone can download and sync all files and folders without any problems)

Mobile client configuration
Client OS (iOS, Android, ...): Android 7.0
Client version: 2.30

Server configuration
Operating system: mac OS X
Web server: Apache (on Mamp)
Database: MySQL
PHP version: 5.6.30
ownCloud version (see ownCloud admin page): 9.1.5
Updated from an older ownCloud or fresh install: fresch install

Hi @Gevian , thank you for reporting this problem. When the downloads you describe fail, is there any error message? If so, could you please include it here?

The message center states (in german):
"Herunterladen fehlgeschlagen. Herunterladen von xxx.pdf konnte nicht abgeschlossen werden"

Important:
When I use FolderSync.App or the Synchronize Ultimate.app on this mobile phone - syncing works perfectly!