Service restart results in hanging tasks (Version 5.6.16)

Hello everybody,
we have a recurring problem when restarting our Elasticsearch service. After restart the cluster does not respond to some API/HTTP requests (search, mget, index, bulk, scan).

What data is needed to help us with this problem in the best possible way? Unfortunatly we cannot upgrade to version 6, because we still have projects depending on multiple types per index.

Thanks in advance for your help

Jochen

Some further data:
https://pastebin.com/9L15RX5x
https://pastebin.com/tiWiaYMf