One account out of four stopped updating, the other three are fine

One out of four is almost always access rather than anything technical, and the fact that the other three are fine is what tells us that - they share the same connection, so if the credential itself had expired you would have lost all four together. The usual causes, in the order we find them: Somebody removed our access to that one account in Google Ads, often while tidying up users and not realising what it was attached to. The account was moved to a different manager account, which looks like a removal from where we are standing. The account was cancelled or suspended on Google's side, in which case there's genuinely nothing to pull. Check the access list on that account first. If we're still on it, reply with the account name and I'll look at what the last attempt actually returned - we log the reason even when the interface only shows you that nothing happened.