Piet, AIFP doesn't know this. So, it must open every add-on.xml file it can find. You can download Development Release 3.2.09(k) from http://stuff4fs.com. It seems to solve such problems for everyone else.
Now I discovered the following:
Each time I try to tick V4 it shows the message and abends.
Then I went trying working with v3 ticketed. I can add AI-aircraft to a plan from my AI-aircraftfolder. As soon as I try to add one directly ouit of the V4-aircraftfolder, the message appears and that aircraft is not added.
Then compiling that v3-flightplan works well.
Then after restart of V4 to let the new traffic.bgl be added to the active scenery, the traffic works perfectly, all 6 planes from the plan show up. See screenshot
Does this help looking into the cause of the error?
Now it seems to be allright. I installed 3.2.09 again, now especially paying attention to Development release.
Then I also made the Prepar3D-folder unaccessable (by renaming).
Then I started AIFP and only the V4 choice button was present and on.
Then opening, editing and compiling existing ai-plans went well.
So end of the search for a solution.
Thanks for your patience and tips.