User Synchronisation: Connection Error

I have installed User Synchronisation on two of my sites and followed all the instructions, but when I attempt to synchronise I get a "connection error" that tells me to check the domain and key.

I have done this (I copied and pasted, so there are no errors).

I have used the DB uninstall and tried again. Same problem.

I have completely uninstalled the plugin and reinstalled from scratch (bypassing the WPMU Dev Dashboard). Same problem.

These are both fresh installs of Wordpress. There are other plugins activated on the master site, but not on the sub site.

Any suggestions gratefully received.