My guess is that something maybe changed with the PPT add in. I'm assuming you are talking about the PPT add in here. Only TechSmith can really say for sure. But I'd say it's likely the version 8 add in doesn't understand something about that version of PPT.
You could try downloading the Camtasia 9 free trial and test. You could also try contacting TechSmith support.
Thanks, you were probably right: Powerpoint 2016 has changed a bit. When I addressed the problem to Techsmith, they immediately gave me an upgrade to 8.3 (from 8.1 to 8.3) and the problem was solved. I have not tried Camtasia 9, but undoubtedly there is no problem for Camtasia 9. Kind regards, Koos