bcooksley wrote:In the case where the icon theme already exists under your home directory, simply replace the icons in it directly. Make sure you replace all the various sizes with appropriately sized icons. After doing that you'll need to re-wipe the icon cache again.

bcooksley, I am positive that this will solve my issue, but still it is a workaround.
Should I open a bug somewhere?

If this workaround works, then yes, you need to file a bug report against the folder thumbnailer.
If it does not solve the issue, then we need to debug it further.

bcooksley wrote:If this workaround works, then yes, you need to file a bug report against the folder thumbnailer.
If it does not solve the issue, then we need to debug it further.

Hi, sorry for the long delay.
At last I had the time to try your suggestion. I overwrote the inode-directory.svg file directly in the theme directory, then I logged out, cleared the icon cache and logged back in.
I couldn't believe my eyes when I saw that this DID NOT fix the icons in Dolphin.
Also, while in the process of overwriting the icons, I noticed that gwenview is even worse: it presents the generic icon even for folders that DO NOT contain pictures.
Here is a screenshot of both Dolphin and Gwenview, showing the same folders.

Gwenview likely uses the previewing system directly - which is why it probably displays the generic icon at all times.
It seems likely that some of the previous attempts to set the icon manually are still hanging around... do you have any customised file associations, etc?

bcooksley wrote:Gwenview likely uses the previewing system directly - which is why it probably displays the generic icon at all times.
It seems likely that some of the previous attempts to set the icon manually are still hanging around... do you have any customised file associations, etc?

How can I reset everything to the default?
Is re-applying the icon theme sufficient?

Please move ~/.local/share/mime/ elsewhere while outside of KDE. After logging back in, please reapply the icon theme, then logout again and remove /var/tmp/kdecache-$USER/ to ensure all caches are completely purged.