"Undefined Variable" Error after Site Migration

Hello!

I have a website based on CodeIgnitor 3.

I want to change hosting, so I made a SQL dump and a backup of the web root, which I both migrated to the new host.


But I'm getting "Undefined Variable" errors, when I try to load the website on the new host.

It seems to me, that for some reasons, code ignitor is not able to load any data from the database, though there aren't any authentification errors. 

I'm lost atm, since the code itself hasn't changed at all and it worked perfectly on the old host.


If someone recognizes this issue or can give me a hint how to further troubleshoot it, I would be very thankful.

Kind regards,
Chris!