Preferences / Folders / Folder Display / Show generic icons for... is turned off.
This problem is happen when I access through symlink. When I directly access to network share, thumbnails are properly displayed.
My symlink direction is Local drive -> Network share.
When I access through Windows Explorer, same problem happens. (On first time everything is normal. After that, every thumbnail is not displaying.)
After some testing, I found this; When I access through library(ex.library -> video), everything seems normal. If I access through symlink(ex.D:\video), problem occures. This happens only on DOpus. On Windows Explorer, no luck.
EDIT: It seems it's Windows side problem(Windows forum link, and there's more.). Still, it's wierd that DOpus 'fix' some part of problem... (Accessing through library actually generates thumbnails normally.)