I was also getting a similar error when using server classloader policy set to single and installing an ear containing a jar file with a version that differed to that used by WebSphere's admin console:
[1/05/08 12:32:58:789 EST] 0000000a ServletWrappe E SRVE0100E: Did not
realize init() exception thrown by
servlet action:
java.lang.NoSuchMethodError: org.apache.commons.validator.ValidatorResources:
method addConstantParam(Ljava/lang/String;Ljava/lang/String
V not found
This stopped the admin console from running, causing much wailing and gnashing of teeth. We solved the problem by deleting the offending ear from the deployed apps directory (and possibly cache/temp dirs) and restarting the server. It complained about the missing ear but at least we could get the admin console back and uninstall the app (or change the classloader policy back).