Cloud Sync to Nextcloud -> all files unreadable/corrupted


I’m trying to sync my files on NAS with Nextcloud (Nextcloud is installed on my root-Server).
Everything seems to work very smooth but all uploaded files are corrupted.
If I upload any file manually everything is fine. I already asked on the synology forum, thinking it is a cloud sync
problem. There I was told that I need to run files:scan --all, which I did:

proton@bigdeal:~$ sudo -u proton php /var/www/vhosts/ files:scan --all
Starting scan for user 1 out of 1 (proton)

| Folders | Files  | Elapsed time |
| 539490  | 764376 | 18:54:28     |

But the issue seems to remain.

Nextcloud.log shows this:

Debug	cron	Run OCA\NextcloudAnnouncements\Cron\Crawler job with ID 13	2017-12-17T12:31:27+0100
Debug	core	OC_Image->fixOrientation() Orientation: -1	2017-12-17T12:31:13+0100
Debug	core	OC_Image->fixOrientation() Image is not a JPEG.	2017-12-17T12:31:13+0100
Debug	core	OC_Image->loadFromFile, Default	2017-12-17T12:31:13+0100
Error	PHP	imagecreatefromstring(): Empty string or invalid image at /var/www/vhosts/	2017-12-17T12:31:13+0100
Error	PHP	file_get_contents(/opt/nextcloud/data/proton/files/var/www/vhosts/temp/oc_tmp_kSbHuS-.jpg): failed to open stream: No such file or directory at /var/www/vhosts/	2017-12-17T12:31:13+0100

I’m grateful for every help.

Best regards