Error 0x80073cf9 when installing or updating apps from windows store

Not sure how it could have happened, but this seems to be a possible solution:

The problem is that the folder AUInstallAgent is missing from the Windows folder. I had this problem, recreated the folder and low and behold all is working fine. Now what I want to know is why the folder disappeared in the first place, because I didn't delete it, and looking around the web I'm not alone....

Microsoft has a support article, Troubleshoot problems with an app, that offers a download of the Windows Store App Troubleshooter which will try to detect and correct any problems with the Store.

This solved it for me.

(I have tried all the mentioned solutions and none of them worked for me. I also tried to run WSReset, uninstall and re-install, nothing wanted to work. I was about to format and reload my PC, when I found one last suggestion to try and it sorted out the problem!)