I thought I would throw my 2 cents into this discussion which will hopefully help some people out because I think I encountered most every problem there could be based upon many other threads on the internet. The Situation: I have installed X plane 11.55 on my system. I had not used it in a while so I had an older version of the GTN 750 on my system. When I booted up X plane, my GTN 750 got stuck with the message "Rebooting" that never went away. Using a previous version on my machine of the "rpnGTN-XPL-Setup file I tried to reinstall that software. I was advised that there was an upgrade and chose to install it. That did nothing to fix my problem. I then tried to reinstall my software with the F1 Purchasing Module which is referred to as the "e commerce wrapper" above from RXP. I received an error that the server was being blocked by a firewall and would not allow me to continue. After a good night sleep, I decided to start from scratch and reinstall both X Plane and the GTN 750 software using the same procedure described earlier. This time the GTN 750 did boot up, however, I did receive the same message as indicated by the comments of others above "Invalid License: Contact Vendor" message. Thanks to all of your instructions in this thread, I uninstalled Reality XP from "C:Program files (x86)" using the "unsins000 file" contained in that folder. I also deleted the "Reality XP folder" from "C:ProgramData" in the same directory. I then used the "e commerce wrapper" to reload the GTN software and this time I did not receive any errors and was successful in reinstalling the latest version. Started the X Plane again and everything works. Lesson learned. Hope this helps and is plain enough language for those, like me, that are not that tech savvy.