You likely have a classloader issue.
Different versions of WebSphere gets packaged with different versions of various files. If a newer version of WebSphere has updated their libraries, the old libraries you are depending upon might cause problems.
What is the name of that jar file your problem class is packaged in? Look on your classpath for similar named jar files. Perhaps you can find another implementation of these class files somewhere else on the classloader hierarchy.
Alternatively, sometimes just switchning the classloader to parent-last, especially if you have the jar files packaged within the web application, will force WebSphere to use your own version, rather than the servers version.
Here's a little tutorial I put together on classloading and application packaging with WebSphere. You might find it helps to explain what is happening with your varous classloaders:
Understanding WebSphere Classloaders: Classloaders Demystified
-Cameron McKenzie