Failed to render "theme/_default/single.html"

When building the website I’m getting the following error:

Failed to render “theme/_default/single.html”: runtime error: invalid memory address or nil pointer dereference

The localhost server shows everything perfectly but when I upload the contents of the public folder, I get the example website with my information on home and contact section.

Any ideas on how I can debug this?

Unfortunately not. That error isn’t the most descriptive one in Hugo, I’m afraid. Did you get more error information besides that sentence?

Else you have to go through theme/_default/single.html and see where you made a mistake with Hugo template code (the stuff between {{ and }}).