I uninstalled 0.75 and installed 0.72. That one worked as expected.
I installed 0.75 over 0.72 (as upgrade). Everything worked well also.
I uninstalled the lastly 0.75 upgraded version and reinstalled 0.75 directly again. Now the image opening failed again.
In my snapshot the XnView can be seen as available choice, but the very first time after OS install it was not present.
Other curiosity is that the upgraded 0.75 version had no icon in installed programs list in control panel, a blank file was displayed instead (like a plain text icon).
After each uninstall (but not during upgrade) I completely deleted XnView folder both in Program Files and in AppData\Roaming.
UAC is off, installed as unique default user with admin rights.
Image type does not matter, is the same with .jpg, .png, .gif, etc.
Sure I can click the "The selected program will always open this file type" (or similar in English), but then I have to repeat for each image type, which is out of question (it is much easy to reinstall 0.72).
Cristi