It was issue internal to the RHEL VM guest OS.
I found reports related to the reboots on RHEL machine in /var/crash.
I began to suspect relation to ipsec, moved connections one by one to the other server (RHEL7) and single connection to the old Cisco ASA, with option "sha2_truncbug=yes" was the one crashing the RHEL8 server. After I got it away, no crashes.
As I am writing now, I noticed both libreswan and strongswan installed on the problem machine, while on the other RHEL7, where I moved ipsec connections, only libreswan is present and it works fine. Possibly crashes could be related to this.
