In short: if your site uses PHP 5, update using the zip file instead of doing a "git pull" on the repository directly. If you use PHP 7 or above, you can use either the zip file or "git pull".
Longer explanation: the code in the GitHub repo contains some code that only works on PHP 7. As Q2A 1.8 was made to support PHP 5 originally, it would be a breaking change so I changed some of it to support PHP 5.
But those changes were not committed to the repo (didn't seem worth it as I'd just change them back again later). So I created the zip file for download using PHP5-compatible code.