Getting constant "Assessment failed: Failed to obtain certificate"

I've been trying to follow the guide on https://timtaubert.de/blog/2014/10/deploying-tls-the-hard-way for setting up SSL properly on my personal domain. However, at some point the SSL validator began being unable to validate at all. I constantly get the "Assessment failed: Failed to obtain certificate" error:

Qualys SSL Labs - Projects / SSL Server Test / udun.dawninglight.net

Any help would be appreciated.