I had the same problem but only after I reinstalled McAfee. McAfee Support said to remove Windows Defender from startup. This works but why did it not cause me a problem when I received my new computer from Dell just a week ago? Did Dell not have Windows Defender in the startup? McAfee Support says Windows Defender and McAfee are not compatibility. So someone enlighten me!
McAfee makes a lot software, some of it may be incompatible, especially a "suite".
We use McAfee "Enterprise Virus Scan" v8.5, and it works ok with WinDefender (although I haven't run WinDef for a while, does it now include a full AV scanner?).
No IT folks I know trust WinDefender very far, they usually use MalwareBytes, and the other usual "free (approved)" anti-spyware programs.
You might want to try Sunbelt Viper, they have a free trial version. It too would be "incompatible" with other anti-virus software, including McAfee.