This seems to be a combination of issues:
A very fresh new ES version (24 hours old…) with a not yet compatible addon, so it seems.
Besides, Zammad does not yet support Elasticsearch 7.6.
(See the end of: Zammad 3.2 is available )
Remove ES 7.6, install ES 7.5.x and ensure to purge it’s data directories (in case Zammad is the only system using it) and start from scratch in ES terms based on our documentation.
You then should be fine.
Edit: I should learn to read: Glad you could solve it on your own. 