Language selection by user's browser settings not working properly for front page

For this website we're using language selction by the user's browser language (next to the path prefix).

But for some reason it would always pick the default language disregarding the browser's language settings (Firefox, Chrome, Safari - all latest versions).

I've enabled the the variable "Default front page" and made the proper modifications in the site information settings, to no avail. I came across this topic concerning a similar issue with a possible solution to it, but it doesn't work for me either.

Checklist:

• I have all the multilingual modules (+ Variable, Variable Realm, Variable Store) enabled, except for Contact translation and Multilingual forum
• The variable Default front page is checked
• FIlled out all the proper links to the translated nodes in Site information, the regular way and also by this method to make sure
• Detection and selection: URL (path prefix) & Browser
• I have Language switcher dropdown enabled. I disabled it to see if it could be related to this issue, but nothing changed (after the cache being cleared).

Is there anything I overlooked?