After W10 reinstall I did just one reinstallation : FSX Deluxe but not Acceleration.
Since I could keep my main FSX folder on my hard drive over W10 reinstall which has been tweaked over the last 8 years, I wanted to rerun it. So the reinstallation mentioned above was made in paralel of the old installation. As you can guess, the idea was reinstalling the files lost on the C drive (FSX.cfg, scenery.cfg, etc).
My old good copy ran good for a month now. I had the error message on startup for a few weeks saying that my copy was not registered but never saw any registration window opening. I no longer have the error message but I'm still limited to 30 minutes and no add-on (FSUIPC makes FSX crash when installed).
I also had a backup of FSX.cfg, scenery.cfg etc which I could put back on the C drive after FSX reintall. FSX was tested at each step.
Searching on the web, I found that there is a way to makes reappear the registration windows but as mentionned, it doesn't works for me.