What @Walter_Palumbo posted isn’t only a nextcloud problem. Please try the following steps from the side Walter posted:
Then after reading this thread and a few other things on the net, I first made a copy of the /etc/letsencrypt folder and then tried a few things.
What finally got it working for me was to copy the .pem files from the mycoolsite.org-0001 folder to the mycoolsite.org one: