I believe it has something to do with Tiled's json. In my version (Linux) it has two types of json, version 1.1 and the current one. I was using 1.1, before gdevelop updated and only today I noticed that Tiled's .json version had also changed! So I deleted all cached folders and everything else from the two apps, and it works again!
Is there any possibility to use more than one layer to create collisions? As well as Tiled objects?