Ok sorry, found the reason. Logstash template was too old. Deleted the old one and restartet logstash and ok.
Just upgraded Elastic 5.6 --> 6.2. Everything went fine and after the upgrade, indexing worked fine.
However, now I noticed, that daily indices have not been created anymore, so indexing stopped at the next night after the upgrade.
Simple question: What is going on and how to fix it? Any idea would be nice, I'm not so familiar with this product.
I also upgraded Logstashes to 6.2. but no help.