Just found out the problem was a comma in my export name...
It used to be "M3U8 Playlist, relative file paths", which was never a problem within actions before.
When I changed that to "M3U8 Playlist relative file paths" the action ran without a problem.
Weird. Is this a bug in v2.89a?
So don't use a comma in your export definition name if you want to use that in an action...
The "v.a." part was not the problem.
Thanks for your input.
I hope people with the same problem can find their solution here.