No traffic over IPsec VPN unless traffic is started from A -> B.

Hi guys,

We're having a strange issue with an IPsec VPN with one of our clients.
The tunnel is established, but does't work as expected.

Only if traffic is sent from A -> B, B can send traffic back.
If A stops the traffic, B cannot send traffic anymore to A after a couple of minutes.

Phase 1 and Phase 2 timeouts are the same on both endpoints. The other settings are all identical too.
Does anyone have an idea what goes wrong here?

Thanks for thinking with me!