it seems I have found two things:
1.) Two of my ESXi Servers did not report any performance data into vcenter as well - I corrected this with restarting the management agents on the ESXi machines
2.) I found a Veeam KB to adjust the values of two REG entries on the ONE Server
HistoryQueryTimeout changed to 3600 (from 900)
RTQueryTimeout changed to 600 (from 300)
After doing both changes it seems to work fine.