I have a similar issue, after upgrading a ClusterXL load-sharing cluster from R80.10 to R81.10. I set both the environment variables as per SK162637 and was able to set the cluster to unicast load-sharing mode.
Remote access Mobile access VPN users cannot connect. They get this error: "Internal error; connection failed" after they are authenticated by Radius server. Logs on SmartConsole do not show any useful information.
In cluster HA mode users do not have issues connecting. Furthermore if I set either gateway administratively down so there is only one active member, users can connect OK. Only when both members are active do users have a problem.
Before the upgrade users had no problem connecting with both gateways active.