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 \  _ {} \;
   `----
   Find searches for all files in the current directory (.) and it's
   subdirectories matching the regular expression ".* .*, which means a
   filename containing any character (.*) followed by a space and then
   followed by any character. On these files, it executes for each
   (-exec) the command rename that looks for spaces (\ ) (This is
   backslash followed by a space, the backslash are used to make the
   shell interpret the space literally, otherwise it will be ignored) and
   replaces it by underscore (_) on all the files found ({}).
2) Start sqlite3 in a shell (insert your db path!)
   ,----
   | sqlite3 /path/to/digikam.db
   `----
   This starts sqlite3 and opens the database given as argument
   (/path/to/digikam.db)
3) Replace space with underscore in the database
   ,----
   | update albums set
   |   relativePath= replace(relativePath, " ", "_")
   | where
   |   relativePath like "% %";
   `----
   This queries the "albums" table in the database and replaces spaces
   in the "relativePath" column, in "relativePath" which contains
   space (actually, I guess the 'where' statement can be skipped as
   the 'replace' statement only works on spaces anyhow!)
4) Done! Start digikam and find nicely renamed albums.