Calendar error when adding an event

[lucabenny]

Hi devs, 

I'm using the open-source, self-hosted version of Canvas LMS installed on a VPS. 

When I try to attach a Module to my course calendar using the "Course Link" connection menu, I get this error:

An error occurred. Please check your connection, reaload the page and try again

Also in my console.log, I see: 

api.js:461 GET https://app.domain.com/api/session 404 (Not Found)

I have attached a screenshot of what I'm doing. 
What I need to do in order to fix this?

Many thanks all!