The new Log4J2 version 2.10.0 creates some really weird issues on WebSphere.
It might be related to the split of the server components into the unreleased tools project(LOG4J2-2064).
I have attached the stacktrace of a WebSphere 9 instance on Windows 7. I hope you can get anything out of it.
It even crashes the server sometimes when I am trying to deploy from Eclipse, but this might be related to the bad quality of the Eclipse plugin by IBM.