Try replacing:

sl_upload($_FILES['quest']['tmp_name'][$this->id], $path.$savefile)

with:

sl_upload($_FILES['quest']['tmp_name'][$this->id], $path.$savefile, null);

Explanation: the sl_upload function is expecting 3 arguments, and you’re passing only 2.

Before PHP 7.1, this only triggered a warning, but since then it’s generating a Fatal Error instead.

Since it worked fine for you before (although, it must have triggered a Warning, which probably got logged somewhere unless your error_reporting is very lax), passing null as the third argument should produce the same outcome as it used to.

It would, however, be a better idea to figure out what that function argument is supposed to be used for, as it’s marked as required (no default value).