This usually means that the server sees your IP as one different from what the master server saw when you authed. In my experience, this happened to me once when I was on a VPN which routed TCP and UDP connections separately, and since the auth server uses TCP and Blockland uses UDP, the server saw me with a different IP than the auth server saw. The main things to check for in this situation would be VPNs, and maybe even phoning your ISP and asking why your IP address may be appearing differently to different services. Additionally, things like mobile phone networks tend to have highly dynamic IP address allocation which means you'll get a new IP address very frequently, which will break your authentication as well.