Hi Denis, thanks for your reply.
The application in question was actually SAP! We were able to spy all of the elements we wanted in SAP (which the SAP spy mode could not capture) on his colleagues machine. In fact, we tried it on a couple of his colleagues machines, no issue. It must have been something to do with the setup of his particular machine, not the end application, etc.
As mentioned, we had seen this a few months earlier with another couple of his colleagues machines, where literally just toggling into AA mode (without getting a chance to try spying anything) would cause Blue Prism to crash immediately. I have not seen this with any other clients (there have been quite a few over the years!), and obviously it only seems to affect a very small number of developers' machines.
I no longer have access to their machine, so can't do any further analysis for you i'm afraid. Guess this one has to be filed in the ""Mystery"" box.