If you scroll back a few weeks, there's a few threads on this. They have apparently decided to start applying demo account inactivity rules to RP, which is irritating. I think @James Morris noted that despite the message, signing in isn't enough. You have to make some server-side change. I've been making a small change to the same customer record every time I get that message, which is irritatingly often.
If your account is already purged, too late for that obviously, but I'd reach out to your AM to put some pressure on the support case.