[rules-users] InvalidClassException: When creating RuleAgent

When I create a new RuleAgent with a package created with the BRMS following Exception is thrown: RuleAgent(BusinessRuleEngine) INFO (Wed Jul 23 12:00:32 CEST 2008): Configuring with newInstance=true, secondsToRefresh=30 RuleAgent(BusinessRuleEngine) INFO (Wed Jul 23 12:00:32 CEST 2008): Configuring package provider : FileScanner scanning: D:\...\mypackage.pkg RuleAgent(BusinessRuleEngine) EXCEPTION (Wed Jul 23 12:00:33 CEST 2008): org.drools.base.mvel.DroolsMVELFactory; Serializable incompatible with Externalizable. Stack trace should follow. java.io.InvalidClassException: org.drools.base.mvel.DroolsMVELFactory; Serializable incompatible with Externalizable ... It works fine with another package created earlier. So it could be something wrong with my fact model. I couldn't find any useful information about the Serializeable incompatible with Externalizable on the web.