Can't drag & drop assets

Hi,

I'm on Unity 5.5.0f3 and I can't drag and drop any assets (PNG, FBX) in the Assets tab. I tried running Unity as admin, not running it as admin, uninstalling and re-installing as admin, placing the project in C:\Users\Username\Unity projects - nothing works. I can only copy-paste files directly in the Assets folder. That's fine, but I just wonder why dragging and dropping just plain won't work.

I'm on Windows 10.