If I remember correctly, Hugo wasn't able to process the template-content combination when I tried starting it after a few months of not using it (Hugo had been updated meanwhile). So I updated the template which didn't solve the issue. Then I tried to find out what changed and it wasn't just a few lines of code, but more fundamental things.
In the end, I just took a completely fresh version of the template and rebuild the documentation by copy and pasting the relevant parts.
For context: Initially I build the documentation in November 2018 and the update happened in April 2020.