Error getting response; null - nothing to do with size

Weird stuff happening.  I'm developing IBM MDM services and running some tests on them using SOAP UI.  There is one new one that I created recently, that when I go to try and run a test of the service in Soap UI it gives the error "Error getting response; null" every time.  There's no actual request sent to the server, it doesn't get that far.  Other services are working fine through Soap UI.

I tried basically dropping out most of the request data, so that it wouldn't even have a possibility of running the service properly, if it ran I'd just get an error.  But still wouldn't get as far as running and getting the error.  Doing the same in other services returns a result which was the error message as expected.

Since I'd cut down the XML so far in the request, I figured I'd just try replacing the request service name with the one that was working.  But that still gave Error getting response; null.  So I then tried putting this new services name into the request for a service that was working.  That one ran and gave the expected error, rather than "Error getting response; null".

So I tried putting the complete request XML for the new service into the Soap UI request that was created for another service that worked successfully.  When I ran that it worked successfully and gave the correct response.

So if I grab a Soap request from a different service, and run this service within that, it works, but if I run it from within the request that Soap UI generated specifically for this request, then it just gives "Error getting response; null" every time.

At least I've confirmed that the error is on the Soap UI side and not with my service in any way.  But this is really annoying.

I've run it in a Soap UI instance on a VM that had it installed ages ago and still has Soap UI version 5.3.0, and I've run it on the Soap UI installed direct on my laptop which is version 5.5.0 and it gives the same result in both.