The error may happen on systems where IBM Java is not installed as System JVM and the TEPS shortcut points to the javaws.exe under C:\Windows\System32 and NOT the one under C:\Program Files\IBM.
The simplest way to correct the issue is to create a new desktop shortcut manually to point to the javaws.exe under C:\Program Files\IBM\JAVA50\jre\bin. When you do that, Java Webstart will prompt you to select the application every time by bringing up "Application Cache Viewer".
To skip this step, simply copy the command line argument to the javaws.exe from old shortcut to the new shortcut. You can get the argument to javaws.exe by right clicking the old shortcut -> go to Properties->Target.
Hope you find this useful.