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Regarding the WMI user access, make sure the password is the same, also the windows user have to be included in the user groups "Performance Log Users" or "Performance Monitor Users", "Remote Desktop Users" and "Distributed COM Users". You also need to enable permissions and services. For more detailed information you can refer to this guide
This has been discussed in our knowledgebase, so you might consider to check out the feedback of PRTG users and additional hints here and here And you can get some more information about the user access settings in this article as well as in this one
We have seen cases where an user is created with exactly all the admin rights in the correct groups and still doesnt work. But when using a local administrator user it works fine. Even with all the same exact access and permissions. This is an issue in the windows side and not from the PRTG perspective but is also something to take into consideration when testing.
Please also see our extensive WMI Troubleshooting Guide it may contain some more information.
If you keep having problems please use our WMI Tester, run it on the PRTG Host (or host of the Remote Probe) and scan against this particular target, usually the "OS-Query" from the "Advanced" Tab of the Tool works best, please send us the Results on a ticket to [email protected]