OK I figured out that the address is missing a "//", meaning it should be "message://%3C..." instead of "message:%3C".
Thought I'd just try to automate a "find & replace" in Taskpaper, but I don't know how.
BTW, Is there a way to change the default list for the Reminders workflow, e.g. if I want it to create by default in "Taskpaper" list instead of "Reminders"?