Hello derHabi,
Welcome to our i-doit forums!
Please take a close look at our knowledge base: https://kb.i-doit.com/display/en/Installation
Linux Mint 18.1 is based on Ubuntu 16.04 which is currently not supported because of PHP 7. Please switch to another GNU/Linux distribution or just wait for i-doit 1.9. The pro version will be published in May.
Greetings,
Benjamin