Hi Silvio,
I suspect something environmental. The REIWA Japanese era was added in between JDKs 1.8.0_144 and 1.8.0_212, but your installation seems confused about whether this era exists or not. How did you install Elasticsearch and your JDK? Can you try a clean installation, ideally from official packages rather than anything repackaged by Debian?
I have tried running Elasticsearch 6.5.4 with JDK 1.8.0_212 (specifically Oracle Corporation/Java HotSpot(TM) 64-Bit Server VM/1.8.0_212/25.212-b10) and do not get this error.