Thanks for the support which I greatly appreciated.
![Smiley :)](https://forum.avast.com/Smileys/default/smiley.gif)
It was probably the connection attempt of my Microsoft account on the PC that had disconnected or something that was left in the browser cache, but I think it's more the first hypothesis.
After clearing the browser cache and logging in to the PC's Microsoft account, no pop-ups appeared anymore.
However, from scans done with Avast and second opinion the PC is clean.