Open Dolphin, go to the Settings menu and select Configure Dolphin. In the window that opens, make sure the General tab on the left is selected, and select the Previews 5t5tab on the right.
It rebuilds the “KDE System Configuration Cache” manually.
Maybe a reboot does the same. If the files are located on a remote location (other disk etc.) you need to adjust the preview settings via “Skip previews for remote files above”, curently set to “No previews”.
I had already rebooted.
Setting Skip previews made no difference.
Running the command from @Aragorn results in:
kbuildsycoca5 --noincremental
kbuildsycoca5 running…
kf.service.services: The desktop entry file “/usr/share/applications/org.freedesktop.Xwayland.desktop” has Type= “Application” but no Exec line
kf.service.sycoca: Invalid Service : “/usr/share/applications/org.freedesktop.Xwayland.desktop”
kbuildsycoca5 --noincremental
kbuildsycoca5 running…
kf.service.services: The desktop entry file “/usr/share/applications/org.freedesktop.Xwayland.desktop” has Type= “Application” but no Exec line
kf.service.sycoca: Invalid Service : “/usr/share/applications/org.freedesktop.Xwayland.desktop”