Sometimes, the Unity editor is in a state where the exporter cannot perform some operations. It can be due to a lot of different reasons.
Closing and reopening Unity resets the scene and the editor to it's saved state, and most of the time it's ok for the exporter code.
It's kind of magical yes but if it happens again, don't hesitate to open the console and post a screenshort or copy/paste the error logs so that we can identify the case and provide a fix if possible.
Thanks for reporting!