[Qt-creator] Debug button not enabling

[Jason H]

I've noticed several times now that the Qt Creator 4.10.1 Debug icon (below Build/Play and above the Build hammer) is staying gray when nothing is happening. I can't seem to get it back...
Things I've tried:
- changing the active project
- running qmake for a project
- mix/max-imumizing the window
- build the project
- rebuild the current project
- close a project
- close all projects
- open a project after closing all

Only thing that works is closing QtC entirely.

Is there a way I can track down what is going on?