"Undefined Variable" Error after Site Migration

that was my first thought.

the original server runs on php 5.6
the new runs on 7.3
first, i tried updating code ignitor system folder to current v3, because i read somewhere, that those version would be compatible with never php versions.
didn't change anything.
so tried to deploy it on another 5.6 stack. but same issue!
i activated log level 4 (all logs), but the logs aren't telling me more then the php error messages in the browser.
Undefined variable warning, then Exception: Call to a member function result() on null
sadly, this breaks the whole site.
it doesn't appear to be related to the php version. so i'm really confused rn.