Thank you Rikard, your log directed me in right direction towards fixing the issue.
Just so you know, it was due to the change in permission to my WP-Includes sub folders and some file, that this problem occurred. My hosting support said, it was due to some files in those locations being flagged during their occasional heuristic scans. They had removed the execute permission, and hence the problem.
I did complete malware scan of the directory and thereafter enabled execute permissions for those location and the Layout editor started working normally, haven’t faced any issue since then.

Will reachout if any additional help needed. Thank you ‘again’ for the prompt support and directions.

Best Regards / Dharam