Monodevelop worked at first, but now it won't launch (error "Invalid action command")

Every time I try to launch MonoDevelop I get the following error:

MonoDevelop-Unity failed to start. The following error has been reported: Invalid action command id: MonoDevelop-Unity.Debugger.DebugCommands.AttachToProcess

When I first installed the Unity package, I could launch it and MonoDevelop. I used MonoDevelop to attach to the Unity process and debug an editor script. It threw an error message (I don't remember what it was) when I opened the attach window, but I could dismiss it and continue and it worked fine. However, after shutting down the computer and starting it up again, MonoDevelop keeps giving that error.

OS: Linux Mint 17.1

I didn't have the regular MonoDevelop installed originally, so I did that after I started having this problem, but it didn't correct it. I also tried deleting the MonoDevelop-Unity-4.0 directory in ~/.config and I see that MonoDevelop recreates it but still runs into the same problem. I've also tried uninstalling the whole Unity package and reinstalling but that didn't fix it either.

Have you guys been successfully debugging with MonoDevelop? Has anyone run into a message like this?