/var/lucee/config/server/lucee-server/context/logs/global.log shows the occasional java.util.ConcurrentModificationException
I don't see that this really impacts the service in any negative way, Lucee's churning out pages just fine. Zac Spitzer told me on slack that there's no obvious way to increase the log level for this global.log in order to maybe get more info and I suspect that increasing the log level for everything in production is not such a good idea.
I guess that as RamCache is involved, this may have to do with Query Caching concurrency.
Debian 9.9, SMP Debian 4.9.168-1+deb9u3 (2019-06-16) x86_64 GNU/Linux, Oracle Java HotSpot(TM) 64-Bit Server VM (build 25.201-b09, mixed mode), Tomcat 8.5.14-1+deb9u3