Hello. I confirm that, with Java 11, a few compatibility changes are needed.
We are working on an update of Lightstreamer, but, unfortunately, this will take a few more days.
Sorry for the delay.

The main issue, which is also the one affecting you, is with the Javassist library, used internally.
Other issues only involve the MPN subsystem and should not affect you, unless you have set <enable> under <mpn> as Y.

So, for now, please update manually the Javassist library under the lib folder.
It is in two places: under lib/core and under lib/mpn/hibernate.
You should remove javassist-3.22.0-GA.jar and put javassist-3.23.1-GA.jar (the related zip files are there for licensing reasons and can be left). You can get the jar, for instance, from Maven.