TaskPaper 2 will not work on macOS Sierra (10.12) and beyond.
TaskPaper 2 was an early adopter of OS X cocoa garbage collection memory management… a technology that went from new and shinny to outdated very quickly. In macOS 10.12 the underlying garbage collection technology has been removed from macOS altogether. Unfortunately this means TaskPaper 2 won’t ever be able to work on 10.12 and beyond.
To solve the problem you’ll need to upgrade (with a discount) to TaskPaper 3. It uses the same underlying format, adds new features such as folding, and generally is a more modern and polished version of TaskPaper.
I’m sorry about this, TaskPaper 1.0 does still work (with a few visual glitches, but generally functions). I just adopted the wrong technology for TaskPaper 2.