Previews generating errors

Hey,
i’ve got a litte problem with my Nextcloud.

When i open a folder with pictures in it, a lot of errors are showing up in the logs.
No previews are generated since the update to Nextcloud 11.
The “preview”-folder in my “appdata_oc5hkfd6e0ae”-folder is empty.

These are the entries in the log:

Error	PHP	touch(): Unable to create file /mnt/NAS/www/html/tobiaswalter/data/appdata_oc5hkfd6e0ae/preview/13725/64-64-crop.png because No such file or directory at /mnt/NAS/www/html/tobiaswalter/lib/private/Files/Storage/Local.php#198	2016-12-31T11:13:57+0100
Error	core	OC_Image::crop(): No image loaded	2016-12-31T11:13:57+0100
Error	core	OC_Image::preciseResize(): No image loaded	2016-12-31T11:13:57+0100
Debug	core	OC_Image::load(): could not load anything. Giving up!	2016-12-31T11:13:57+0100
Debug	core	OC_Image->loadFromFile, could not load	2016-12-31T11:13:57+0100
Error	PHP	fclose() expects parameter 1 to be resource, boolean given at /mnt/NAS/www/html/tobiaswalter/lib/private/Files/Storage/Local.php#218	2016-12-31T11:13:57+0100
Error	PHP	fread() expects parameter 1 to be resource, boolean given at /mnt/NAS/www/html/tobiaswalter/lib/private/Files/Storage/Local.php#217	2016-12-31T11:13:57+0100
Error	PHP	fopen(/mnt/NAS/www/html/tobiaswalter/data/appdata_oc5hkfd6e0ae/preview/13725/2048-1152-max.png): failed to open stream: No such file or directory at /mnt/NAS/www/html/tobiaswalter/lib/private/Files/Storage/Local.php#216	2016-12-31T11:13:57+0100
Error	PHP	filesize(): stat failed for /mnt/NAS/www/html/tobiaswalter/data/appdata_oc5hkfd6e0ae/preview/13725/2048-1152-max.png at /mnt/NAS/www/html/tobiaswalter/lib/private/Files/Storage/Local.php#211	2016-12-31T11:13:57+0100
Error	PHP	touch(): Unable to create file /mnt/NAS/www/html/tobiaswalter/data/appdata_oc5hkfd6e0ae/preview/13731/64-64-crop.png because No such file or directory at /mnt/NAS/www/html/tobiaswalter/lib/private/Files/Storage/Local.php#198	2016-12-31T11:13:57+0100
Error	core	OC_Image::crop(): No image loaded	2016-12-31T11:13:57+0100
Error	core	OC_Image::preciseResize(): No image loaded	2016-12-31T11:13:57+0100
Debug	core	OC_Image::load(): could not load anything. Giving up!	2016-12-31T11:13:57+0100
Debug	core	OC_Image->loadFromFile, could not load	2016-12-31T11:13:57+0100

How can i fix these errors? Every other functions are working.
Thanks for your help!

Hi,
i fixed it by renaming every folder (Bilder Umbau --> Bilder Umbau_bak) and renaming it back (Bilder Umbau_bak --> Bilder Umbau). Now preview-images are generated again.