How to prevent Sabre\DAV\Exception\Forbidden on sharedhosting?

Hello everyone,

how do i prevent these upload errors “Sabre\DAV\Exception\Forbidden” on my shared hosting (domainfactory)?

Windows Client: 2.3.3
Nextcloud; 13.0.1
Nextcloud is installed as a subdirectory because of the one provided ssl certificate of my hoster.

Example Log-Windows Client:
23.03.2018 15:29:59, pathtofile/file.pdf, D:\Cloud,Error transferring - server replied: Forbidden (Sabre\DAV\Exception\Forbidden)