EDIT: After completely clearing my cache, both the browser and the "app" are working normally again.
I just realized that I was always able to open the editor on the browser offline, but that I cannot through the "app" (icon on my desktop).
When I open through the browser and then proceed to "Open in Construct 3" all is fine though.
Could I've missed something? I'm using beta versions and maybe the download got "stuck" in some version?