Thank you for your guesses and intention to help.
The cause of the failure is the same banal one that I ran into a couple
of years ago - the name of the directory.
I am so misfortunate that my user account was created originally with
the hungarian special character Á.
Moreover, this folder included the characters ü and ő. For God knows
why, windoze allows creating directories
using these, however a lot of applications cannot handle them properly.
This is the case now.
I just simply copied the pictures to a folder that's path uses only
standard ascii characters - and it works!
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