"Error: File has invalid header" occurs when the current stp configuration file (on OSX by default "bcman.stp") or the current mru configuration file (default "bcman.mru") doesn't start with the line "[BC Manager]" (without the double quotes).

I don't have your OSX version, but in my version the dialog box stating "Error: File has invalid header" offers two options: "Press OK to ignore and risk data corruption" and "Press Cancel to kill the program."
When I press Cancel, OSX aborts BC Manager, and the error occurs again the next time I try to start BC Manager.
However, when I press OK, OSX allows BC Manager to execute its own termination routines. These routines include the creation of updated, (hopefully) correct configuration files, so BC Manager starts correctly the next time.
So if your OSX version offers the same choices, try "OK" and see if BC Manager starts again.

If that doesn't work:
In Finder, execute "Go to Folder" from the "Go" menu, and enter "/users/UserName/.config/mountain_utilities/bcman" (without the double quotes), then delete or edit the offending stp or mru file.
(Beware: the ".config" folder is hidden, so unless you've configured Finder to show hidden folders, you won't see ".config" in your home folder.)

Hope this helps,
   Mark.