a 500 error occasionally appears showing the error message: "Could not initialize class org.jenkinsci.plugins.ssegateway.sse.EventDispatcherFactory"

A reload of the page always solves the issue.

The logs show the complete stacktrace to be:

java.lang.NoClassDefFoundError: Could not initialize class org.jenkinsci.plugins.ssegateway.sse.EventDispatcherFactory
{{ at org.jenkinsci.plugins.ssegateway.sse.EventDispatcher$SSEHttpSessionListener.sessionDestroyed(EventDispatcher.java:384)}}
{{ at jenkins.JenkinsHttpSessionListener.sessionDestroyed(JenkinsHttpSessionListener.java:63)}}
{{ at org.eclipse.jetty.server.session.AbstractSessionManager.removeSession(AbstractSessionManager.java:775)}}
{{ at org.eclipse.jetty.server.session.AbstractSession.invalidate(AbstractSession.java:326)}}
{{ at org.eclipse.jetty.server.session.AbstractSession.access(AbstractSession.java:278)}}
{{ at org.eclipse.jetty.server.session.AbstractSessionManager.access(AbstractSessionManager.java:173)}}
{{ at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:164)}}
{{ at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)}}
{{ at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)}}
{{ at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)}}
{{ at org.eclipse.jetty.server.Server.handle(Server.java:499)}}
{{ at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311)}}
{{ at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)}}
{{ at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)}}
{{ at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)}}
{{ at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)}}
{{ at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)}}
{{ at java.lang.Thread.run(Thread.java:745)}}