I am having the exact same problem...
I'm sorry, but I can only repeat and confirm this doesn't happen. I've checked several times (not that I wasn't sure, but I always check when a problem is reported) and there are no problems: the Addon Manager correctly gives the message when P3D is in Full Screen, and the registration dialog correctly appears when P3D is in Windowed mode.
Are you sure you are not using those FSX-P3D "migration" utilities that play with the .exe version numbers, so the Addon Manager is confused about what product is really running, so it's looking into the wrong memory address to detect the Full screen mode ?
I have installed P3D 1.4 in a clean windows installation and the GSX installer without using the migrator tool witch I own but cetrainly not used during installation.
There's no other explanation than the migration tool left some trace of its use, even if it wasn't active. That would be the only possible explanation why the Addon Manager thinking is running into a different sim, most likely FSX.
This is really frustrating..I cannot understand how uninstalling GSX doesn't solve the problem.
It's fairly obvious why you don't solve the problem by uninstalling/reinstalling GSX: the problem doesn't have anything to do with GSX so, keep working on it, will only lose your time, since the problem is caused by something else, most likely the migration tool, so you should work on that one instead.
Please I need your help because I cannot register and use my GSX product!
You can't register your product because some other external product has altered your P3D configuration, so it's not running correctly as it should.
Do you have FSX installed too ? If yes, you could simply register from FSX: the activation is NOT stored in the sim, it's stored in your Windows registry so, as long as you have both P3D and FSX running on the same Windows installation, you can share the same activation.
EDIT: I would like to add that during the installation, reading the text running above the compeletion bar I read the text ''Python support file XPOI....was not installed"
This has been explained many times already on the forum, and it's NOT an error: it simply a progress indication, saying that the updater is skipping the update of support files for products you don't have installed.