I tried to un-set the ‘block messages from users not in your contact list’

But very time I reopen the server settings window, I see that it is ticked again?

You’re quite right – it is storing the change on the server correctly, but then misinterpreting the server’s response. You should find if you sign out and then sign in again the option won’t be ticked.

I’ve fixed this for the next release (which handles configuration significantly differently).

Thanks