Quick Update: the problem had to do with the indices not permitting writing/setting them to read-only due to storage problems. See my answer here: Elasticsearch Docker: flood stage disk watermark [95%] exceeded
After manually updating the settings, the indices kibana_1 and kibana_task_manager_1 can be deleted by using
curl -XDELETE 'http://localhost:9200/.kibana_1' --header "content-type: application/JSON" -u elastic -p
After that it will work.