Okay,
So I have managed to contact our Platinum Elasticsearch support.
After a session of debugging the issue we have figured out that the issue was due to a configuration change in the Kibana -> Advanced Settings -> meta fields, value was changed from:
_source, _id, _type, _index, _score
to:
_source
Which caused a bad request like (which returned Bad Request 400 response):
https://hidden-host/kibana/api/index_patterns/_fields_for_wildcard?pattern=waf-*&meta_fields=_source
Instead of:
After resetting the meta fields configuration to default everything was working normal again.