This was a good thread. Thank you for all the info. I had respective issue for last 3 to 4 days. All the suggestions above are in place & yet user-specific check box was greyed out.
It is 'coz there was a enhancement which is used to stop users from changing standard layout. It should have been for a single transaction code but it was reflecting for all.
Lesson learned: Simple thing can be tough to identify. Have to look into alll the impacting code bits. Its like stakeholder (SH) management. Cannot ignore ANY single SH Image may be NSFW.
Clik here to view.