High Sierra - Failure w/thumbnail cache

Hi,

after I upgrade my iMac to High Sierra, I've got a problem with Finder and Thumbnail icon previews.

I stored many short video clips on my network-drive (Synology NAS) . Everyone was named and enhanced with a special thumbnail, created with Image2Icon.app, So I can find them easylie.

With Sierra, the Mac shows the preview images propably, even after restart of the mac or NAS.

At the time I upgraded to High Sierra, after opening the folder, Finder.app first only shows placeholder icon for multimedia file (like mp4, m4v, wmv). I tried to set and reset "show icon preview" in view options of every folder. Therefrom Finder load every icon propably, but needs much time (it seems, they load the hole file in background, and search for an image). But after restart of the Mac, Finder lost all the preview thumbnails, and restart looking in every file and load the thumbnails again.

Before High Sierra, macOS / OS X stored the quickview thumbnail file in /var/folders/.../com.apple.QuickLook.thumbnailcache in an file named: thumbnails.data..

This filesize didn't change anymore, and got only a few KBs. On Sierra, this file has got >500MB.

Does anyone know this problem and can provide help?

Thanks a lot.