I've stumbled multiple times upon PSD images, by some artists that were probably exported from Photoshop or Clip Studio Paint, which both the dopus viewer and built-in converter fail to load / convert respectively.
Here's an example of one of those files:
psd-sample.rar (2.9 MB)
Both clip studio paint and photoshop on my computer have no problem opening them.
Also resaving them in the same format from both applications generates files readable by dopus.
The original files open flawlessly in IrfanView and HoneyView as well so only DOpus refuses to do it.