You've run diagnostics on one return and you attempt to run it on a second return.
You receive an Access Violation error box similar to the one below
Open that second return again
Edit the return and retype something, such as the middle initial or first name.
Save the return
Run diagnostics and it should not give the error
Also note that you can create the efiles without this error by accessing the TOOLS menu option, then "Make Efiles"
This will be resolved with 31.04 program update