I noticed that all FBX animations I export from 3DXchange 7 seemed to be freezing at a certain point.
It turns out the animations were being exported in full, but 3DXchange 7 was doubling the total track length, so the last half of an animation was always empty. For example, if I export an animation that runs for 3 seconds, the FBX animation will be 6 seconds long, with the first 3 seconds being the animation, and the last 3 seconds being nothing. As a result I'm having to manually halve the length of each FBX animation I export.
While this isn't a production blocker, it's quite inconvenient and frustrating.

I've tried:
a. tweaking all of the different 3DXchange 7 export settings - nothing works
b. tweaking all of the different Unreal 4.16.1 import settings - nothing works
c. re-importing the newly exported FBX animations back into 3DXChange 7, and the problem persists, meaning the problem is being caused by 3DXchange 7 and not third party software
d. uninstalling and reinstalling 3DXchange 7 - it didn't work.