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It turned out, that there were missing FW Rules between the CARP interfaces. Did you proof that?

Yes.  We add a Pass rule to the CARP interfaces on both firewalls but it disappears on the slave, sometimes within seconds after creating it.

We are experiencing similar issues.  I'm running OPNsense 18.1.13-amd64 on two identically configured HP DL360 G5 (OFW-A and OFW-B) configured for high availability.  Interfaces are named the same with the same assignments as follows:

WAN (em0)
LAN (em1)
CARP (bce2)

OFW-A CARP is plugged directly into OFW-B CARP with no switch involved.

In order to get pfSync to work we had to add an Allow rule to the CARP interface on OFW-B.  As long as that rule is present, the web interface on OFW-A is responsive.  However, the allow rule on OFW-B disappears all the time breaking the HA and the killing OFW-A web interface performance.