I have now downloaded GSX directly from FSDT and installed it. This was somewhat more successful because the add-on manager now appeared in P3Dv4.5 under the Add-on menu.
Unfortunately, the activation is aborted with this error message:
The Cuatl scripting engine hasn't started. This is usally caused by an antivirus that has blocked it. To fix this problem: Exit from the Simulator. Configure to the antivirus Exclusion list. launch FSDT Live update or reinstall the software.
I quit P3Dv4.5 as recommended, reinstalled GSX again, put the Addon Manager in the exclusion list. This measure was unsuccessful because the window with the hint appeared again.
Then I uninstalled GSX again, turned off Windows defender and reinstalled GSX again.
However, this measure was also unsuccessful. I cannot activate the software.
Ciao Horst