The action "export cover to file", creates an empty file (file with no extension and with size of 0 bytes) when the album name has a ":" in it.
For example, if I try to export the cover of a song from the album "Awake: The Best Of Live" by the band "Live", with the string for the filename being set up like this: "%artist% - %album%" . Then the result is the empty file, called "Live - Awake". (note that the part of the album name that comes after the ":" is not there)
This doesnt happen if the filename string is changed to only: "%artist%". In this case, the exported cover turns out fine, just with a filename I dont want. I think the ":" should be switched for a "-" in these cases.