7.8.0, a change to optimise the speed of loading variables from the Logstash Secret Store could cause Logstash not to be able to start when the feature was used in conjunction with multiple pipelines. This has now been fixed, and you can read the details here: #12236
Logstash is not yet compatible with JDK 15.
While we are working to support JDK 15, we encourage you to use supported JDK versions (8, 11 or 14). See Java (JVM) version for details and the Elastic Support Matrix for the official word on supported versions across products and releases.