m is not a registered protocol

Ever since I upgraded to the 1.5 RC1 version of Portable Thunderbird, I get the message "m is not a registered protocol" when I open the app. I suspect the "m" refers to the drive letter of the thumb drive that PT is running on, but I'm not sure.

If I move the Portable Thunderbird folder to the C: drive, I still get "m is not a registered protocol" when I launch it.

I've tried googling this, but haven't found a solution.

This didn't happen with 1.0.7. It only started with 1.5 RC1. Any ideas on what might be causing this or how to resolve it?