branchoffice - headoffice s2s suddently stopped working

We have a XGS 136 in the headoffice and and XGS87 in the branch office.

We have setup a site-to-site VPN and the channel is activated and connected successfully on both devices.

It was all working find as expected.

Since 3 or 4 days suddenly, we can no more connect from branchoffice to headoffice, but all is ok from headoffice to branchoffice.

We did an upgrade ofthe firmware on both devices to SFOS 20.0.1 MR-1-Build342 around the time when the problem started.

But I have looked at everything and tested policies from both sides to the other side. All tests are ok and accepted by the correct rules.

Still, we cannot connect to servers in the headoffice from the branchoffice. Pings are not working from branch to headoffice, but the other way round.

THis is very strange, would be grateufl for any input.

thanks

ks