Hmm...cert manager by default is only accessible by local machine admins. Is the user account that this device is associated with not a local machine admin? Or is there some magic you've done on the other computers so that cert-manager is accessible by non-admins?
You're correct about the user store not requiring UAC - I had assumed you meant the device cert store, not the user cert store. I should have thought with my brain why you'd be asking the question in the first place
Has the machine been running normally otherwise? Do other MMC windows (like the device cert store) open without unusual behavior?