No firewall or DNS issues 902 UDP is open and working fine. I am actually able to ping the vCenter server from ESXi during the issue, and stay connected to both the ESXi server via SSH and to the VMware Server via RDP as well.
The issue duration is 2-12 minutes, after which everything reconnects just find and stays connected until the next occurence of the issue (2-3 times per day).
In the instance today, one host started as disconnected (and stayed disconnected for 5 minutes), then 2 host (for another 2 minutes), and then everything came back online.
Regards,
Lukas