Owncloud doesn´t show some files with german Umlauts

Hey guys,

it´s really weird but my Owncloud doesn´t show files containing german Umlauts in just one single folder with some sub folders.
Everywhere else I can see al files, isn´t that silly? If I rename the files and rermove the umlaut the file appears after a refresh.

EDIT: I tried following: I removed the Umlaut manually. The file appeared in OC and I renamed the file within the OC WEB UI and the file is still there, So I hope there is some kind of rescan possible? I tried this command before but it didn´t solve this issue: sudo -u www-data php occ files:scan --all

It is a local folder added as external storage.

OC: 9.1.3
OS: Ubuntu 14.04.05
Nginx PHP-FPM latest versions

What kind of external storage?

It is a local folder on the Ubuntu Server itself and I added this local folder as external storage,