That wasn't going to be the source of the problem, as it occurs even with the user role editor plugin disabled, and wouldn't account for why it occurs on your server but not my local server.
Anyway, after emerging from a deep dive (which is tricky to do remotely) I'm pleased to say I identified the problem, and have fixed it on your site.
The issue is actually fairly straightforward, and arises from a flaw in our code that handles this process... when using https (which my local test site does not, hence I couldn't see the issue).
So I have made an edit directly on the relevant Access plugin file, so you should be good to go for now, and we can hopefully close here.
(In the meantime I am escalating this to the developers; the fix is quite simple, and should be included in the next Access plugin update so you shouldn't need to do anything special.)