I feel a bit dumbfounded, but I can’t figure how do I enable the main left sidebar menu.
Any hints?
I’ve just spun up a completely new install.
From an admin account, go to the Site Settings, and select ‘Sidebar’ in the navigation menu setting.
Are you using a trial version of Discourse or a self-hosted one?
Can you paste a full screenshot of the desktop view of your forum?
If I go to your forum, the page tells me it’s version 2.8.14. Discourse is now at version 3.1. You should do an upgrade.
<meta name="generator" content="Discourse 2.8.14 - https://github.com/discourse/discourse version 2.8.14">
Did you follow the standard install? A fresh install should always install the latest version of Discourse.
I’m using the Cloudron.io Discourse source, since we have a corporate install of that one. Wasn’t aware that they are behind. But makes sense now.
“One-click” Discourse installations are usually not recommended here.
One of the main reasons is that we don’t know how they configure their stuff, so it’s not a standard install, and thus you’re less likely to find free support here on meta if you encounter issues. 
If you just started your forum and if you can do that, I’d strongly advise you to install Discourse following the official guide
Welcome, Damien!
Basic hosted plans, including trial, ship with the sidebar by default.
The old hamburger menu isn’t available anymore and will be discontinued, and thus the enable new notifications menu setting, which relies on the old hamburger menu, isn’t available anymore as well. 