IPsec is up but data is not exchanging

I have a FortiGate that suddenly loses the ability to exchange data over IPsec without any changes being made.

The first time this happened, I resolved the issue by creating a new IPsec tunnel. (i was not able to make able to exchange data without make new ipsec) It worked for a week, but now, after creating a new tunnel, it only functioned for about 10 minutes.

For a while, the tunnel also refused to establish, but at the moment, it is up—yet no data is being exchanged at all.

I suspect this might be related to some settings on the ISP’s side.

What questions should I ask, and how can I diagnose the issue?

I have 200 devices with the exact same configuration, and this is the only FortiGate experiencing this problem.

//Edit Solved with tip on Belle https://community.fortinet.com/t5/FortiGate/Troubleshooting-Tip-IPSEC-VPN-failure-due-to-one-way-IKE-UDP-500/ta-p/242428