Hello zsolt,
thanks for the reply! I tried what you suggested, but had still the same symptoms, i.e., I can see a "locked" folders contents but I cannot open any of the contained files. At least now, when I double-click on such a file, I get the prompt to enter the password for unlocking the folder. Still, it is not optimal that one can see the folders contents by name, file type and (sometimes) the icon preview.
I am not sure what you could see in the Console (I am not an expert), but I attach a screenshot showing some lines from the Console that might be relevant.
I am not encrypting any folders - I guess I feel that data loss is much more likely when encrypting...
Cheers,
-J. P.