Hello there, @ichi80 ...
Interesting! After you set the folder to the permission of "Only available with link", how did you obtain a link for that folder? Did you right-click on the folder name on the right side of your "FIles" screen to copy the link? That is what I did, and my link looked something like this:
https://SchoolName.instructure.com/courses/123456/files/folders/FolderName
...where SchoolName is your school's Canvas environment URL, 123456 is the Course ID number, and FolderName is the name of your folder in your Canvas course.
I tried pasting my URL from the course "Files" area into a discussion topic within a group I had in my own sandbox course. After I saved the discussion topic, I clicked on the link, and it took me directly inside the folder where I had an image stored. So, in my testing, things worked as expected. (I also had my "Files" screen hidden from student view...if that matters.
Can you maybe detail your process a bit more or even record a quick screencast to post here so Community members can see exactly what is happening? Maybe a screenshot or two would be fine if you're not able to record a quick video screencast.
Looking forward to hearing back from you soon!