L2VPN uses the same protocol and ports (TCP/443) what the SSL VPN-Plus is using that is why you cannot use the same on one ESG.
I tried changing the port of the SSL VPN-Plus to 8443 but I was still not able to enable them both at the same time ...
OK, I should have been more precise here. This is using the same service on the ESG. You cannot enable both at the same time. That's just how it is. Maybe it will change later.