As you may be knowing that sometime SCCM client fails to register with SCCM server and because of which the system fails to connect and get any patches and as well as any application and AV definition. This specially occurs to mobile devices who are working on laptop, or connecting using RAS(Remote Access Services).
Point 1: Verify ClientIDManagerStartup.log
Point 2: The existing solution is to stop the 'SMS Agent Host' service, then delete the system file that starts with '19C5...' under the directory 'MachineKeys' under 'ProgramData' and then restart the 'SMS Agent Host' service.
Once followed above step issue got fix. If still having issue Click Here for further reference.
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