I contacted to managewp but no avail, they insisted there is some issue on hosting side & it was very hard to believed that suddenly 9 different hosting account had the issue? After long frustration & checking everything, my co-worker Wyatt rectified the issue. Cloudlare plugin was the culprit. Further info can be found here
https://wordpress.org/support/topic/bugs-in-1321-crash-wp-site?replies=10