Possible Root Causes, by error code:

0x800706ba (The most frequent error)

Wmic may also report an empty output. For more details, about this symptom, see Discovery: How to test the WMI connection with wmic?
The error may include "The RPC server is unavailable"
Cause: The required ports documented in the section "WMI" in this page are not open

0x80070422

The error may also include "The service cannot be started, either because it is disabled or because it has no enabled devices associated with it."
Cause: The WMI service on the target host is stopped

0x800706b5

The error may also include "The interface is unknown"
Cause 1: NT AUTHORITY\SERVICE is not in Administrators Group

Solution: Re-register WMI components and add NT AUTHORITY\SERVICE  to the Administrators Group.

Cause 2: Some WMI components are corrupted

0x80041004

Wmic may also report "Failed to access Win32_NetworkAdapter : Failed to retrieve batch of results : 0x80041004 : Provider Failure". For more details, see the article below:

Cause:
Discovery: Failed to access Win32_NetworkAdapter : Failed to retrieve batch of results : 0x80041004 : Provider Failure.
 

0x80070005
            The error may also include "Access is denied"

Cause 1:  Incorrect domain and/or user and/or password.

Cause 2: UAC is enabled. It has to be disabled in certain cases (see "UAC" in this page). See this article.

Cause 3: The credential does not have the required permissions as documented here. See the section "Granting permissions".

0x800706be (Found in the proxy debug logs)

The error may also include "The remote procedure call failed"
Cause 1:  This page suggest some checkpoints/causes/solutions for this error.
Extract: “… occurs when a RPC query times out”
Cause 2:  See also this page:
Extract: “… can be caused by a number of different factors including software incompatibility or corrupt files”

0x800706bf 

The error may also include "The remote procedure call failed and did not execute"
In this case, the scan of Windows 2012 server ends with NoAccess, and a credential test may fails with "Remote Access via Windows Credential Proxy xxxxxxx failed".  

Cause:  See this page

0x80041003

The error may also include "Access denied"
Cause: the scanned device is not in the TrustedHosts list of WinRM (Windows Remote Management)
Solution: Execute the command below on the proxy:
winrm s winrm/config/client @{TrustedHosts="*"}

0x80070721
            Testing with wbemtest results in "A Security package specific error occurred"
            Cause:  Time difference between the Outpost and the scanned device.

For a detailed step by step guide on how to troubleshoot Windows WMI discovery failures please see following video