Zywall USG 300 ipsec config issue

Question

We have a Zywall USG 300 which dials ipsec to a Server 2008 R2 server.

This was working until recently when any attempt to communicate with the server sees the following in the logs.

error
IPSec
SPI:0x0 SEQ:0x0 No rule found, Dropping packet [count=2]
internal-ip
external-ip
ipsec

There hasn't been a change on the network... (at least that we're aware of) to explain it stopping working

We have Zywal routes set up

1. Anything incoming on the VPN routes to our LAN
2. Anything from our LAN destined for the external network routes through the VPN

We have firewall rules set

1. from the external network to the public IP endpoint => allow
2. from the external network to the public ip endpoint => allow
3. from the LAN to the external network => allow
4. from the external network to the LAN => allow

Networking isn't my strong-point(!!) and I'm not sure where to begin troubleshooting this...

Any advice appreciated...