Trying to investigate this a bit on my own.
If I open the Output Log window, every time I try to start dragging the ReloadRifle animation over to the Montage timeline, it puts up this warning each time:
LogRHI:Warning: OpenGL debug output extension not supported!
This is on a Mac. I read some previous posts related to the message saying it was a bug fixed on Windows by Microsoft, but this is Mac. 
Not sure how it’s related other than it looks like it’s trying to report a message but then pooping on yet another problem where it cannot even show me the message it wants to?