Sure there isn't anything in your Jetty stderrout log? $JETTY_HOME/logs by default? The critical exceptions will usually fall to that log. But to answer your question about webpasswordsafe logs, edit /webpasswordsafe/src/main/resources/log4j.xml before building as described in the Administrator Guide. Admittedly the default configuration is very Tomcat-centric, thus the use of catalina.base environment variables, change those to full paths that make sense on your server.
I was playing around with Jetty a bit and found a possible issue with the way it handles JSESSIONID that could cause an issue with the CSRF protection in some fringe cases I'll look into. But I can't be sure this is what you are running into without more info from logs.
Thanks,
~Josh