I have a similar issue, except that I'm spying Excel's UI. (I've tried 2-3 other applications as well, with the same results)
Blue Prism should be connected to the correct process, as there were no other Excels open. Also, I have tried all the Application Modeller modes.
The spying operation seems to be detecting the same UI elements in AA mode as it detects in Win32 mode - however, it is giving the above error message for me as well.
The problem seems to be machine-specific, and it must have been broken because of a Windows update :
the same operation with the same application worked on that machine before (I was returning to a former process, which worked - however, all the actions relying on AA elements broke)
on a different machine, the elements can be spied with AA mode without any issue - however, the elements spied this way cannot be detected by BP on the former maching.
Are there any Windows updates known to break AA mode? Based on a quick googling this kind of error affects other automation solutions as well.