I'm playing back the MP4s in VLAN. I've played all the footage I can get my grubby little hands on, the 720p60, the 1920x1080 footage, all of it works except for the tennis footage, which plays back strangely, as per the attached png of a section of a screenshot.
Both clips are exactly the same. They produce that weird pixelation effect constantly.

What does this look like to people? It is some clash of codecs?

That looks kinda corrupted. I have the boring cars footage on my server again try them instead.

ex.bolanski.com

That looks kinda corrupted...

Is that something caused by my pc or the download or something else?

Is that something caused by my pc or the download or something else?

That I can not say. Try downloading an other clip and see if it works.

Is that something caused by my pc or the download or something else?

It must have been corrupted on its way to you; the same clip plays perfectly here.