Update 3:

I've tried the same export, but now with Graphics Acceleration set to Custom with the following settings:

GPU for display and image processing: ON
GPU for export: OFF

This makes the export problem free and uses 100% of CPU capacity.

Pretty sure the issue lies somewhere in the GPU export

I also have the impression that the GPU-accellerated export is not reliable yet. Additionally, at least on my systems it runs way longer than the non GPU Export. This a a pure Lightroom issue (not LRTimelapse).
Therefore it is something that you should report to Adobe. The more users report those issues, the better.