The error is in an async thread, so perhaps the webapp has started to undeploy.

If the webapp is undeploying and if Tomcat is before v7.0.54 and if the class in question was never loaded before undeploying, it may cause such NoClassDefFoundError. See https://bz.apache.org/bugzilla/show_bug.cgi?id=56321

So it would be interesting to know your Tomcat's version.

If that's not the issue, it is indeed strange that a logback class throws NoClassDefFoundError: ch/qos/logback/classic/spi/ThrowableProxy.