I also had this issue
I am using microsoft windows server 2003 R2
I changed the composite file into a non composite file and didn't get the error after that.
When I changed back to composite - the error was back
I ended up removing the suite that was causing the trouble (it was a temporary suite anyway)
Until smartBear find out what is causing this problem, try to save a copy of the project and remove the suite that causing trouble if its not causing you too much extra work.
Note to smartBear -
In my case this had happen after I cloned one suite a few times and made changes to the clones - I did that the day before I updated my Ready!API, after which the error started showing.
One other thing you guys can try is look in the error log and see if there's a stack trace and if so what it says. It may help you track down the source of the error.
One of my project files became corrupt once and I had to open up the file in an xml editor and manually delete code that had somehow become duplicate creating duplicate properties which caused a failure when loading test steps. Very annoying until I read the stack trace and manually edited the project xml code.
