I have a working instance of nextcloud 14 and wanted to install a letsencrypt certificate.
I followed the guide https://doc.owncloud.org/server/10.0/admin_manual/installation/letsencrypt/apache.html and apache itself was working fine and showing all of my other subdomains allright with the letsencrypt certificate. A previously (without letsencrypt) working instance of nextcloud would show up but as soon as I try to login, it would throw an error like " openssl_pkey_get_details() expects parameter 1 to be resource, boolean given" and not log me in. My mobile phone would still be able to login.
I finally had to take out the “SSLOpenSSLConfCmd DHParameters /etc/apache2/dh4096.pem” and without it, it is working all fine now. I tried one full day changing config back and forth and the solution was only not using this single line! I hope this helps someone.