Try to open "https://YOUR_SERVER _NAME:443/client/clients.xml" using the web browser on Veeam ONE Server machine. The error you've posted means that there is something wrong with the client.xml
file we are receiving from the vCenter Server (or ESX(i) host). I would also recommend you submitting support case
with our technical team, as your issue might require extra troubleshooting which is not possible to do via community forums.