I know this is an old thread but in case anyone else is looking for an answer, I found that I got this exact message when I did not fill the subnet info for the Client VPN in. Once I filled that in, the page saved as normal. Why it gives this erroneous and misleading error is a bit of a mystery to me though.
Yesterday it happened again on site-to-site VPN and Traffic Shapping rules. There is no announcement from Meraki about this issue. I tried the workaround presented by MrChips but did not work this time.