Sprwt runs an automated defense against carding/account-takeover bots that log in from datacenter and proxy networks after creating accounts from ordinary residential IPs; it also blocks an account that logs in from an unusually large number of distinct IP addresses in a short window (a sign of rotating-proxy automation, not a real person). Occasionally a real customer on a VPN, corporate network, or hopping between wifi/cell towers trips one of these checks. Use the red "Unlock login" badge on their profile to whitelist them — this both unblocks them and skips both checks for that account going forward.