An interesting problem that has appeared recentley is that I've seen the RuleAuthor fail with an exception:
java.io.FileNotFoundException: Could not find RuleHome.uix.uix
on a few occasions recently. It is possible to get to the login screen, but once you have logged in, this error appears.
It turned out; that this was caused by that the Oracle HTTP Server (OHS) was started by using apachectl instead of via OPMN. This led to inconsistency between the OHS to OC4J mapping in the mod_oc4j mappings. Could be worth keeping in mind...