Did you do they following:
For the WebCenter Sites Contributor interface to load properly, edit the setDomainEnv.sh file under the <domain_home>/bin directory and add the commons-lang-2.4.jar file, used by WebCenter Sites, to the PRE_CLASSPATH as follows:
From the installation manual...
make sure your classpath contains the ... commons-lang-2.4.jar. you can find this Jar under your CS webapp web-inf/lib folder.
I updated that jar file in classpath of startWeblogic.bat which solved my issue.
Hope you should resolved that issue by now, if not try this approach.