I didn't read the upgrade material carefully enough, and imported the upgrade framework to my AX 2009 application, before activating multisite in all companies. Now there's an issue with one of the tables of the framework, preventing me from activating multisite.
At this point I could of course uninstall the upgrade framework again, but my laziness is in the way of that solution.
Adding the table to the list of tables the multisite activation wizard shouldn't concern itself with seems to fix the problem:
Use this tip at your own risk.
I'd however like to consider myself a bit excused, when the upgrade guide contains the following statement: