Out of curiosity, where did you install the addon manager to?
The default location: C:\Program Files (x86)\Addon Manager
It's NOT required anymore to install into the sim and, in fact, it would be best if you don't do it, so you'll be able to uninstall/reinstall the sim in the future, without affecting the Addon Manager installation.
I manually created an "Addon manager" folder in P3D and copied in the bglmanx.dll and bglmanx_qwings.dat files from my new addon manager location and now the 757 works. My guess is it doesn't like that it can't find those files in the old location.
?? Of course it would never work like that! Why you haven't used the proper Stand-Alone Addon Manager installer ? The installer does A LOT more than just creating a folder and copying files, it must register the dll with the sim, in their proper location, change configuration files, set permissions, etc, etc, etc.
Never, ever, do things manually, when there's installer available.
Now, it would be best if you remove all the the things you added manually, then download and install the current version of the Stand-Alone Addon Manager.