Google Apps: Error 1000 when changing SMTP settings

Do you have Google Apps on one of your domains, left over from the days when they gave it away for free to organisations of fewer than 10 users? I do.

Ever tried to tick this box and got an "We are unable to process your request at this time. Please try again later. (Error #1000)"?

A search suggests that many people on "legacy" (free) Google Apps have run into this, but naturally Google aren't going to help unless you become a paying customer.

Fortunately, I stumbled across the solution: after ticking the box and before clicking Save, change one of the other settings on the page as well. I used the catch-all e-mail setting (and then changed it back later). Google are correct that the setting change above takes a few hours to propagate to all your GMail users, but it has now started working for me.