Looks like all usable spots are already in use. Some decorations might provide additional spots.
If I try to drag a pot in my Atrium it gets rejected - no message, just thrown out. Is that message 
just a little misleading or am I doing something wrong?
The same happens in the Wintergarden, but I don't get the message about decorations there.
I think it means like the ghost - it gets a whole different spot other than what the pots and/or plants do. Does that make sense?
Yes, I see what you mean, except that I don't have a ghost. 
You're probably right. I took it that if you placed a pot, you could then put a plant in it.
Thank you. 
I get this message in my Winter Garden when I try to add another pot:
"Either this decoration does not match into this garden or all positions are allready ocupied."
I just tried again. I don't get any message in the Wintergarden. 
I currently have 3 pots that are unusable. I think you earn them for different milestones in your garden, but you'll have to wait to use them until we get more spaces added into the Wintergarden or they make the pots useable on future slots added to the Atrium or add another gardening space that uses them.
