I honestly can't remember what "Stage 3" was. Is there any indication on the screen of exactly what the installer is currently trying to do? If there is, that might offer a clue to what the problem is.
Also, can you please read this post and give us full details about your computer, as detailed in that post.
I checked Help, PDF Manual, browsed questions on this forum and received email from Magix telling me to search all the areas you mentioned.
History - I installed the trial version of PMD on 4/27/2020. With two days left of trial activation, I purchased PMD 13.1.1.12. Downloaded and began installation. I did not remove trial version as I thought the purchased PMD would overwrite it. The installation started at 3:23 pm and downloaded about 1/3 the file. Showed it was downloading Stage 3 of 6. At 12:30 am, after no progress, I deleted the download process.
I think I see now how/why this problem has arisen! Ideally what you should have done was to simply enter your supplied key, following purchase, to the Trial version; doing so would have automatically converted it to the full version.
However, I think you need to follow this procedure to get a fully-functioning system back. It would appear that the trial version is still at least partially installed and needs to be removed first. So, please download and install the free version of Revo Uninstaller, point it at PMD and let it find and remove ALL files etc. connected with the program. (If you have any other Magix programs installed, just check carefully everything it will tell you it is removing!)
Then reboot your computer and attempt the reinstall again using the link to the downloader file in the email you received. Once finished, enter your serial key and email address and everything should be fine.
In the unlikely event that the above does NOT work, there is another method, involving use of this Microsoft Utility to completely remove all traces of the program (make sure you follow very carefully the instructions for its use), then reboot the computer and try the installation again.