I have come across this before and based on one of the posts by Lars, it seems there is issue with soapUI 2.5.1 in saving Preferences. If you are using soapUI 2.5.1 Pro ( I guess you will come across this in non Pro version as well ). The issue is this:
1. It is possible that you opened up the Preferences and saved preferences. Even if you don't save the preferences shall change for that run of soapUI.
2. You will wonder- What happens when you change preferences - well, there is property on HTTP Pref tab that assigns HTTP Version for all soap requests, it seems that property is by default assigned to 0.9 in soapUI 2.5.1. So when you change preferences this property which would have been 1.1 for you earlier gets changed to 0.9 and all your requests start failing.
3. You can change this property explicitly to 1.1 and save prefs and your problem is solved.
4. I chose the option not to use soapUI 2.5.1 till any of major bugs are resolved as I think 2.5 is pretty stable. Its upto you what option you guys choose.
btw, I am not soapUI support person. I am one of you guys using the product. However, thought since I have come across this before that I shall share this with you in case it saves you any trouble.
hope that helps,
Sachin