I do (or thought I did) have .NET 3.5.
The slave machine used to be my old main machine, but I recently upgraded to some new hardware: it worked fine on the old machine before.

Anyway, I tried reinstalling the .NET 3.5 framework (just in case), and still getting the same problem.
I also tried deleting Octopus and redownloading/starting again from scratch, same problem.