I recently experienced this issue and it broke down to the fact that I had selected the preference to "create backups of projects before saving" and left the "backup location" field empty. Turns out that ReadyAPI/SoapUI, instead of saving project backup in the same location of the original project file (which would make the most sense, imho...) defaults to saving the backup somewhere in the ReadyAPI/SoapUI install location (or some other unexpected location). Turns out sometimes that location is not writable by the current user (in my case, because only root user had write access). And ReadyAPI/SoapUI doesn't bother to check whether write permissions exist, it just tries to save, and if that fails it send back the error message without parsing it into an intelligent form.
Suggestion to the ReadyAPI/SoapUI devs: add a try-catch statement around the code trying to create the backup, and if it fails due to java.io.IOException, let the user know it's an issue with the backup, not with the original; also do an active check on whether write permissions exist there; also, in the preferences window, add some detail that lets the user know where the default backup location is. Also, maybe change that default backup location to the project-dependent ${projectDirectory} location (or whatever that internal property is, I forget); or maybe add that as an alternative---it can default to either the project-dependent ${projectDirectory}, or some fixed location.
