NB: THIS IS ALTERING YOUR DATABASE /AND/ YOUR FILENAMES AND CAN
POTENTIALLY DESTROY YOUR DATABASE /AND/ FILESTRUCTURE! ONLY ATTEMPT IF
YOU UNDERSTAND WHAT THE COMMANDS DO! AND KEEP A BACKUP HANDY!

1) Rename all files from the command line. This replaces all spaces with
   underscore the current folder and all subdirectories. Execute it
   repeatedly until no error message appears (the error message appear
   because folders are renamed before subfolders)
   ,----
   | find . -regex ".* .*" -exec rename \  _ {} \;
   `----
2) Start sqlite3 in a shell (insert your db path!)
   ,----
   | sqlite3 /path/to/digikam.db
   `----
3) Replace space with underscore in the database
   ,----
   | update albums set
   |   relativePath= replace(relativePath, " ", "_")
   | where
   |   relativePath like "% %";
   `----
4) Done! Start digikam and find nicely renamed albums.