Art error probably. Any way we can see the expanded error message? Or can you make a save file on that event screen (not usually a good idea, but might help here)?
Another thing I noticed is that when you try to influence the mercenaries FS with neo-imperialism, the food FS stuff is in media and I'm not sure what the second upgrade was but that is in the food FS section. As shown in this screenshot.
There is indeed no $activeslave defined, which is probably why you're getting an art error; this is due to there not being any specific slave linked to these random FS events. I should probably set it to define a random slave, which would fix these errors. Also going to check the mercenaries FS to make sure media and food didn't get places swapped.
Looks like the descriptions got improperly assigned to categories when updated SpecialForce to the new system. Both bugs should be fixed by !7830 (merged)