Here are some possible causes of this error:
1- Check to make sure struts.jar is in your WEB-INF/lib directory
2- Check to make sure that the commons-xxx.jar files and the struts.jar file are for the same version of struts, meaning that they all came from the same struts download package.
3- Make sure that there is not another struts.jar or one of the commons-xxx.jar files being loaded by another Class Loader. This could happen if there's a copy of one of these jar files in some common area used by your App Server.