Which client are you using? Basically from the 1st of August the new Let’s Encrypt Subscriber Agreement is in effect. If the client tries to “agree to SA/TOS” using a link to the old one, it will fail with this error. You could probably just update or patch the client in question to make it work again.
It sounds like you’re encountering the case @roland described in Boulder issue #2102. There’s a patch in the works to relax this behaviour in the future but it won’t likely land in production for at least 1-2 weeks.
Like @leader suggested it is probably best to update whichever ACME client you are using such that when it sends the updating contact email address it also sends the most up-to-date agreement URL.
Can you create a Certbot issue describing the original email update problem you encountered? I think this is a discussion that would be better to have there