Corrupted thumbs after uploading to Ugreen NAS Photo App

Hi, I just registered to this forum because I've stumbled upon your post. I have exactly the same issue but can't find any solution.

My case may be a bit different than yours: I don't use the Ugreen App to sync my Photos because often times it just didn't sync in the background. I use the FolderSync App on my Google Pixel and have a WireGuard VPN to my home network to be able to sync photos when I'm out and about. I only get this corruption when the files get synced while I'm not connected to my network directly. So when I'm home I can delete the affected photos from the NAS (I also have to delete them from the recycle bin) and then trigger another sync with the FolderSync App, then the thumbnails / previews seem to be fine. Interestingly I don't always get this error, only approx. 50% of the photos are affected.

I don't know if this is a Problem with my VPN or UGOS or FolderSync or some combination, but it has worked flawlessly just until a few weeks ago. I didn't change anything deliberately or at least I don't remember changing anything in my config.
As you mentioned the file displays correctly when viewed in full view, so I'd think it has something to do with the thumbnail creation. Further more I don't think you got the same fiddly setup with the VPN and FolderSync, but still have the same issue, so my setup probably also isn't the issue.

I'd really appreciate some suggestions or ideas.

I remember this problem from the days of Windows 7.

You could adjust the size of the preview images in the settings. If there wasn't enough RAM, the previews were only partially loaded and displayed. They were only slowly reloaded when you scrolled. However, since I haven't done anything with images on the NAS yet, I can't really say anything about the cause. But it could be a direction to pursue if you are familiar with the program. Or if something like that can be set up at all.

It think I have found a solution for my case. It seems that the thumbnail creation got triggered and completed even though the jpg files weren't even transferred completely.
In the FolderSync App there is the option of a "temp-file scheme". Temp-file scheme means files are transferred using temporary name, and upon completion of transfer is renamed to the final filename. While being transferred the file type is "tacitpart". This means it only gets converted to jpg once fully transferred, so the thumbnail also only gets created when the file is fully transferred.
With this option turned on the thumbnails are fine. At least for the 20 photos I just took as a test.

I think so to. I hope thats it.