Disabling the "Allow Incoming SIP Messages from SIP Proxy Only" parameter appears to resolve the registration failure issue however this might not be a suitable fix for most because the chances are, if you had this parameter enabled, you were likely a victim of those pesky SIP scanners! Thankfully all is not lost though, downgrading your devices firmware to the last stable release (v18.104.22.168) is an option until Grandstream fix the issue.
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