I did a bit of experimenting over the last couple of hours and I've found a 'workaround' or sorts. Browse to data modeler and create an object with the same name as the object that was causing the issue. You can then add all the fields as they previously existed minus the circular reference. It's not an ideal solution but it gets me out of a bind.
If anyone has a better workaround I'd love to hear about it.
In the latest version, you should have a versions drop down where you can see previous versions and open them and if necessary restore an old version. This of course requires the data modeler to open correctly (which doesn't seem to be the case here).
Could you open a jBPM JIRA so we can reproduce this issue and make sure the modeler can still open correctly?
If you go to the Authoring -> Administration perspective, you can open a repository and then revert the repository (as a whole) to a previous state. Note that that will remove any commits up to the point where you revert (including changes in other files and other projects).