Solved! Go to Solution.
Ok - just in case anyone ever has a similar issue. Turns out this was because of the MailChimp/Mandrill extension. We weren't using Mandrill so had no API key in there, (or possibly an incorrect one) - so it was just a matter of disabling Mandrill and then everything worked ok.
Ok - just in case anyone ever has a similar issue. Turns out this was because of the MailChimp/Mandrill extension. We weren't using Mandrill so had no API key in there, (or possibly an incorrect one) - so it was just a matter of disabling Mandrill and then everything worked ok.