Shiro login not possible if zeppelin.server.port <> 8080?

Apache Zeppelin 0.9.0 preview1 including basic Shiro auth works fine as long zeppelin.server.port specified in conf/zeppelin-site.xml is 8080. However, I would like to use another port (e.g. 9000) for my zeppelin site (experimental localhost setup). When I change the port in zeppelin-site.xml, Shiro auth is no longer possible: the small indicator to the left of the "Login" button on the Zeppelin welcome/login screen is no longer green, but red and login is not executed no matter what credentials I enter.