Change cert.pem to fullchain.pem
Most likely that fixes it. If you have a much older version of Apache, that won’t work and you need an additional SSLCertificateChainFile configuration pointing to “/etc/letsencrypt/live/goblinsama.eu/chain.pem”
The extra certificates in these files form a “chain” between your certificate from Let’s Encrypt, and the roots trusted by web browsers. Long ago nobody did this, but these days almost all certificates everywhere use a chain because it controls the risk for the root Certificate Authorities.
When your server doesn’t send a chain, the browser must improvise, so sometimes it will work and sometimes not. So you should always configure the server to send the chained “intermediate” certificates as above.