Ok, I completely uninstall SoapUI from my PC, then rebooted, then re-installed it without the hermes plug-in (that was checked by default). From there, I created a brand new SOAP project from a WSDL, then I went to the generated test case request. I clicked on the green + icon to add an assertion, then I had the exact same problem. The pop-up window opens, all categories on left side are displayed but when I click on any of them, nothing happens. I am stucked with the type Contains that is associated to the Recently Used category.
Then googling about the error in the log, I saw this: http://stackoverflow.com/questions/31409653/classnotfoundexception-after-updating-to-soapui-5-2-0
I did what this guy did and yes, no more error in the log, but still incapable to add assertions in my project. New log attached.