We tested further and it might be a problem with our domain configuration. The Veeam One server and the hyper-v guests are members of domain A, but the hyper-v hosts are members of domain B. We have now established a trust between both domains, but still it doesn't work. From the DataProvider_HyperV.log we can see following error:
"ERROR Failed to create PSDirect connection (guest1.domainA on host1.domainB) System.TimeoutException:
at LoadHelper.PSInvoker.invokeImpl(Action`1 i_CmdAdd)
at LoadHelper.WmiOverPsInvoker..ctor(ConnectionHolder i_Parent, BaseLog i_Log, String i_ServerAddress, Credentials i_ServerCreds, String i_VmDnsName, String i_VmGuid, Credentials i_VmCreds, String i_Namespace)
at LoadHelper.ConnectionHolder.GetWmiDataProvider(String i_ServerAddress, Credentials i_ServerCreds, String i_VmDnsName, String i_VmId, Credentials i_VmCreds, String i_Namespace, Boolean i_TryDirect)
We have also enabled logon auditing on guest1 and at the same time of the above error message we can see a failed logon in security event log. From that error we can see, that the Veeam One server always tries to connect to guest1 using domainA administrator account regardless of what we have entered in guest OS credentials.
Maybe that helps to identify the issue.