Dear all,
it seems that I have found a workaround how to fix photo sync in windows.
Currently I have iOS 11.4.1 | Windows 10 1803 x64 | iTunes 12.8.0.150 x64 and since months photo sync has not worked. I got the synced folders on my phone but those were empty.
I have changed / adjusted following on windows and now all my photos were synced completely!!
Editing Thumbnail View in Windows Explorer
In Windows Explorer Options remove the checkbox "Always show icons, never thumbnails"

Source: https://www.thewindowsclub.com/thumbnail-previews-not-showing-windows
2.) Using NOT the default new "Photo-App" in Windows 10. Instead use the "old" one Windows Photo Viewer.
Here is the link how to: https://www.cnet.com/how-to/how-to-get-windows-photo-viewer-back-in-windows-10/
Also I have assigned this to my default viewer for jpg, png and other graphic files.
3.) Change visual effects / thumbnails on windows
Control Panel --> System --> Advanced --> Visual Effects --> ACTIVATE "Show Thumbnails instead of icons"

Source: https://www.thewindowsclub.com/thumbnail-previews-not-showing-windows
4.) Open every photos folder --> View large Icons
I opened every folder with photos inside (those which were expexted to sync) and changed the view to large icons. With this every photo thumbnail has been created.
5.) After the above steps I started a sync in iTunes again and finally all photos were synced completely!!
So it seems that itunes needs windows thumbnails for syncing all photos!
Finally, after moths!!, I got photos on my iPhone again. Hope this helps someone else!
Regards
Wilhelm