There is an old reference to this
here, but no solution was posted to the forum.
I
strongly suggest you send in a bug report
and attach a copy of your project. This seems to be a fairly rare problem, so Sparx might not have had the opportunity to see it 'on the hoof' yet. This might have made it impossible to diagnose.
In the meantime, my
guess (and just a guess) is that you are having some kind of MS Jet error. This would only apply if you are using an EAP file, not a DBMS repository. If so, see if cleaning up the file makes the problem go away.
First, make a copy of the EAP file, so you don't risk trashing the original. Work on the copy. Open the project in EA and from the main menu choose Tools | Manage .EAP File | Repair .EAP File. If the process completes without errors see if you've solved your problem. If not, try Tools | Data Management | Project Integrity Check. This might highlight some internal data issues, perhaps allowing you to repair the problem.
Regardless of whether you solve the problem using the above steps, you should consider sending in a copy of the original (broken) project file. Let Sparx know what steps you take (on the copy), what error messages you received, and whether these steps solved the problem.
David