I moved all picture from drive/Folder Fotos to new drive Fotos_1 (because i have not enough space anymore and installed a new SSD) When I get started digiKam, he ask my for the new Location of the albums and migrate something. Now, everything is working fine so the Folder preview Thumbnails are missing. for each folder i set the folder preview picture. Did you now where this information is stored?
Thanks for helping.
Same as https://forum.kde.org/viewtopic.php?f=255&t=141003