-Right click on your tree
-Click on the 2 arrows on the bottom of the list
-Check/uncheck Create new folder.
Thank you for the tip, but it didn't fix it.
Now, going along your tip, I just went to the first item in the feature manager, the part itself -> right clicked -> Tree Display and found that "Show Flat Tree View" was checked.
Unchecked and everything went back to normal.
Also found that Ctrl + T is assigned for that as shortcut. It was surely that!
Hope it may help someone!
Aha, stupid. That's it indeed