Not sure what do you mean with "new P3D installer". Are you using the one we released a couple of days ago ?
If yes, there wasn't any need to download the Addon Manager, and the problem simply shouldn't happen, and I cannot reproduce it.
If no, you must download and install the current KMEM installer.
And, you don't say which version of P3D you use, so be sure you use the latest one ( 3.4.18 )