Thanks for the reply.
I also tried to set 54 vCPUs for the main server and 2 vCPUs for the secondary server sometime ago, but it didn't made any difference. The error happened after a few days of uptime.
I'm now trying with 24 vCPUs ( less than the number of real cores, ignoring hyperthreading ). Let's see if this helps somehow.
Just a note: the bare metal CentOS server shows/use 56 CPUs and works fine with it.