I've now redownloaded and reinstalled the game and the problem still occurs exactly as before.

Hi Avi - I've not opted in to the dev build and I'm 99% sure the servers I was trying to join weren't dev builds either but I'll go check.

Edit - confirmed, this is happening on servers that describe themselves as being both dev build and non-dev bulid.

OS is Win7 64 bit, forgot to select that whilst reporting the issue. I can be moved into an empty slot by a server admin, or join an empty slot if I am hosting myself, but not join an empty slot myself on a server I am connecting to.