suddenly Cluster Management with iManager does not work any more...

We have 2-server clusters 2 on OES2 SP3, 1 on OES11, 1 on OES11 SP1;
iManager runs on OES2 SP3 or OES11 SP1.

If I go to Clusters, My Cluster in iManager I see the turning circle for
quite a long time, then the yellow question mark. After that iManager is
dead, every page returns a error 500 or 503, tomcat needs to be restartet.

Any ideas?