Well, on the ESXi hosts I'm running NTP and the timezone is set for UTC (default). Howevre, I found a couple of hosts not running NTP. This has been corrected, but the vCOPS were not running on any of those servers.
On the vCOPS servers, the timezone is set to "Europe/Paris".
There are no firewall restrictions, as it has been working before January, 20th.
Regards,
Lars