I am a new UGreen DXP4800+ user and I love the Photos app but I am noticing that some of the thumbnails that the app generates are either blank or just partial. If I download the image I get the full image, so there’s nothing wrong with the underlying image file. I have tried re-indexing the photos folder multiple times but it hasn’t made any difference. Is there any way to regenerate the thumbnails for individual photos or fix this problem?
As a last solution 
Try clear cache of thumbnail folder :
Using SSH :
sudo rm -rf /volume1/@thumbnails/*
Then restart it will regenerate the whole again
I found the option to regenerate thumbnails and that seems to have fixed the issue.
Control Panel → Indexing Service → Regenerate Thumbnails
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