Moderator Notice
This is an issue with the optional Open With plugin, not the calibre ebook-editor - so I'll merge this thead into the OW plugin thread.
Some time back Kovid added an Open With feature to the calibre base. IMO, its much easier to use -- its configured by file type, doesn't require separate icon images, on Windows it makes use of the File Explorer Open With list.
Some users have had problems integrating the OW plugin with some 3rd party programs on some platforms. An upcoming calibre release (not 4.0 - later) will probably require some adjustments to all plugins. Given there's a built in alternative for the OW plugin, and the plugin has no active maintainer, users of the plugin might want to consider using the built-in OW.
IIRC something was added to the OW plugin to run arbitrary programs, the Job Spy plugin, which is actively maintained, has a couple of tools (Basic and 'Advanced') to do that.
BR