Copy files form external storage to internal group folder throws error


if I try to copy files from external storage to internal one I get at the first try an error (“could not copy ‘file’”) and no files are visible, but at the second try I get the message “could not copy ‘file’, target exists” and the files are visible at the new location.

The first attempt takes a moment to complete, so it seems, that the system is copying, but the second one is finished without any delay.

The log show each time this message:
“Fatal, webdav, Sabre\DAV\Exception: path needs to be relative to the system wide data folder and point to a user specific file”

The external storage is connected by the administrator with ftp (I also tried webdav).
The internal folder is inside a group folder. Copying files to a standard folder works fine.

Nextcloud version 15.0.8
Group folder version 3.0.2
‪Ubuntu 18.04.2 LTS‬
PHP 7.2.19

Is this a known issue of group folders or some kind of misconfiguration of nextcloud/server?
I searched the web for this problem, but didn’t find anything up to now.

Thanks a lot for your help!