@Marc1 it sounds like you've been through all the basic troubleshooting and rebooting equipment just in case, you have your two MX appliances connected and they are online in Dashboard and have good connectivity out of each location, but the site-to-site VPN tunnel is not coming up. I'm assuming when you go to Security & SD-WAN > Monitor > VPN Status, you can see the VPN peer listed in the table but perhaps it's all red with no connectivity and the usage/latency numbers might be 0. At the top of that page, look over the connectivity graph, is it all red or green or alternating, and if you hover over different pieces of it, is there anything about being unable to connect with the VPN Registry? I'm assuming you have the local LAN subnets included in the VPN on the site-to-site configuration page. Also make sure you did not leave the local LAN subnets at the default on both MX appliances, otherwise you would have the default 192.168.128.0/24 in both places so I'm assuming they're unique subnets. Also take a peek at Security & SD-WAN > Monitor > Route Table and make sure that looks as you would expect. If it seems like everything is configured as it should be and the S2S is just not coming up, I'd go ahead of open a ticket with Meraki Support, they will be able to check on the back end if and where the appliances are getting stuck building the tunnel.
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