Error when logging into the UME

I am having problems logging into my UME. This is an ABAP + JAVA install on the IBM iSeries. I've got a total of 4 SAP instances with the J2EE Engines installed. This works on all excpet one. When I enter my userid J2ee_admin, it throws an 'unknown error'. I've got the trace level turned up, and the only thing I see in the default trace file is:

(UNKNOWN_ERROR) not found in properties files-UNKNOWN_ERROR

[EXCEPTION]

#1#java.util.MissingResourceException: Can't find resource for bundle java.util.PropertyResourceBundle, key UNKNOWN_ERROR

Any ideas?

Thanks,

Diana