Try enabling this....Normally things will work when "Unset" but it's possible to disable the ability of Software from being able to invoke the Ctrl-Alt-Del.

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If that is not it, I would suspect some type of needed exclusions on AV or Security Software.....

Start with a base image w/o any Software or AV of any type and then test Remote Control.  Perhaps Register the device to a Container using a Key passed when installing the agent to install into a Folder that sets the "Bundle" and "Policy" Spoke/Feature disabled.  Then test remote control.  RC policies will still function in this state.  This way you are not pushing any policies or bundles.

If that works.....Perhaps turn on the Bundle spoke and then start adding stuff to see when it breaks.  If it does not....turn on Policies.....

If you have a domain....leave the PC out of the domain during initial tests....if its still not failing...then add it to the domain to see if a Domain GPO is having an impact.

I'll give those a shot.  I did have an issue with AV before, but neither of these have anything on them yet.  One was recently re-imaged, as of Friday.  That's the one I remoted into, then restarted for an update and couldn't login again, which prompted the thread.;  Thanks for the ideas and I'll report back.