Guest services external guest authentication - Invalid session ID

Jack617

I have been trying to use guest service external authentication on a TZ470 running sonic OS 7, however when the POST is done to the mgmtBaseUrl with its additional parameters e.g. https://192.168.1.157:4043/externalGuestLogin.cgi?sessId=6ec799e1d44e6dc9bd9e02ca6a5d2847&userName=testUser&sessionLifetime=1800&idleTimeout=600 but no matter what I do it keeps giving me the 253 invalid session ID error in the xml response:

<?xml version="1.0" encoding="UTF-8"?>
<SonicWALLAccessGatewayParam xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="http://www.sonicwall.com/SonicWALLAccessGatewayParam.xsd">
	<AuthenticationReply>
		<ResponseCode>253</ResponseCode>
		<ReplyMessage>Invalid session ID</ReplyMessage>
	</AuthenticationReply>
</SonicWALLAccessGatewayParam>

I've seen mention of the session ID query parameter using a different name so as well as sessId I have tried sessionId, sessionID etc. but this has no effect.

Is there something I am missing as to why it keeps reporting this error?