VPN keep up and down after migration from UTM110 to SG115

Current situation is randomly receiving notification email from sg115, it's over twenty mail per day, telling the VPN is going down, and the VPN is up again after 1 minute. like this:

VPN connection 'from openvpn-xxxxx.xxxxx.xx' [SSL] using Site-to-Site is down.

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System Uptime      : 5 days 6 hours 51 minutes

System Load        : 0.14

System Version     : Sophos UTM 9.509-3

Please refer to the manual for detailed instructions.

VPN connection 'from openvpn-xxxxx.xxxxx.xx' [SSL] using Site-to-Site is up again.

--

System Uptime      : 5 days 6 hours 52 minutes

System Load        : 0.13

System Version     : Sophos UTM 9.509-3

Please refer to the manual for detailed instructions.

for the OLD UTM 110, the Site-to-Site VPN is working normally without any notification email, once the VPN is up, and will not keep go down and up. 

I just backup the configuration from OLD UTM 110, and then import the configuration to NEW SG115, do i did something wrong?

thanks for you help.