500 Internal Server Error
I managed to fix this problem myself and I wanted to share this solution. Hopefully it helps someone.
Problem description
I installed Account Right 2020.4 SE. It was a fresh installation with no previous versions on this machine.
When sending a GET request to the localhost:8080/AccountRight it sucessfully returned a list of companies.
When sending a GET request to more precise endpoint like localhost:8080/AccountRight/[Compant ID]/Employees the server kept returning 500 Internal Server Error.
Solution
The installation file must be missing some components. I uninstalled 2020.4 then installed and uninstalled all SE versions from 2019.1 to date. I noticed that as a result of this whole process some firewall exceptions were created and also SQL CE 3.5 installed. Finally I installed 2020.4 and everything works fine. Obviously the 2020.4 installation file is missing one of the prior required components.