Hi Ipstenu,
Finally got this resolved.
'NextScripts: Social Networks Auto-Poster' plugin was the culprit. Have contacted plugin support for assistance.
This is how we figured it out :
- ajax calls were returning response, but were also giving 500 Internal error. This means nothing wrong with WordPress core files.
- all sites on our network were having the same issue. So some things was hooking on to WordPress ajax after we get the response. This had to be a network plugin.
- After deactivating network plugins one by one, zeroed in on this plugin.
I know this should have been our first step :(
Thanks you for your help and support :).