IPSEC Site to Site with certs no longer works.

I've had a site to site VPN link working ok between a few sites, for years.

They used certs to authenticate and all happy.  The certs are not expired.

Now, the sites connect but no traffic flow, (cannot ping etc).  Tried recreating with new certs, still no good.

Recreated using RSA keys for auth, works fine.

Anyone got any clues on this?

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