vchanger, Bacula, and "ERR=Unknown error during program execvp"

 After upgrading Bacula to 5.0.3, I started getting an error message when attempting to load a tape:

ERR=Unknown error during program execvp

I suspected it was a problem with vchanger, as it couldn't load/unload tapes, either. After looking in the disk that vchanger happened to be using, I discovered that drive0 obviously had a tape in it, but the file loaded0 was empty. I determined which "tape slot" was empty, added the name of the slot to the file loaded0, and unloaded it. Everything worked after that.