Also, it seems like I can no longer add new SOAP Request test steps. When I click Add Step -> SOAP Request, I am prompted to "Specify name for new step". I specify the new step name and click the OK button. Then I am prompted to "Select operation to invoke for request" and click the OK button. Finally, I am prompted to "Add Request to Test Case" and click the OK button. The dialog box disappears and nothing happens (i.e., No new test step).
For the NoSuchMethodError exception: I had the same problem after upgrading from 5.5 to 5.6. I found that after the upgrade there were a mix of older and newer files in SoapUI's lib folder. For example, the update appears to have added the httpclient-4.5.5.JAR file, but it did not remove the earlier version httpclient-4.1.1.JAR file.. There are several JAR files that were updated, but the update left the older versions in the folder. The classloader is picking up the older file, instead of the newer file. I removed the older versions of duplicated JAR files, and this seems to have corrected the problem.