Random port during VPN IPsec communication

[kircios]

Hello !

I apologize in advance for my English, which I think is average.

When we have done the maintenance of a server, we encountered a problem with our ipsec VPN.
I'm posting a topic here because I really don't know how to correct the problem.
The diagram of our network is as follows:

On the PFSense router, we have a VPN ipsec which is connected with one of our customer.
During 2 years this vpn worked well. But after a restart of the host server, the tunnel has been cut.

The screenshot attached shows us the current status of the VPN.

We can see that the vpn is established but, because of this problem, we can see that in the screenshot, we don't receive any packets.

We have identified 1 cause. Indeed the exchange for the vpn is done from port 500 to port 500. However the customer sees us arrive with a random port that changes with each connection request. However, in the pfsense logs, I can clearly see that the communication takes place with the port 500.

To try to correct this problem we tried to reinstall an old version of the router which was functional, but the problem persists.

I don't know if you've encountered this problem before, but if you had any ideas it could help me enormously.

Don't hesitate to ask me if you need any other information and thank you in advance for your help.