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We just had this as well on half of the switches we upgraded. All switches in the stacks did upgrade, but the environmental variables were erased from some. Setting those brought them back online. We went from 15.2(7)E10 to 15.2(7)E12 Bug ID CSCwo569...
Using root to create the local account did not help. Save button is still greyed out on all 3 big browsers when trying to change the password after creation.