So apparently, I have access to PSO2 again, and I wanted to install it on my new machine.
The official sega install launches and downloads the latest launcher, tells me it's done, then I press the button and... windows 8.1 tells me that he cancelled the launch of pso2updater.
I have admin rights and gave all kinds of permission to folder access, made a root directory for PSO2 etc, but nothing works. I couldn't find anything remotely connected to my problem on the official Microsoft support pages, and the official PSO2 page doesn't even mention installation problems.

So... if anyone has a lead... I'm all ears.
But first, some sleep.