This can happen with aggregate logit if there is a problem with the utility estimation and the solution has failed to converge. The problem can be caused by one or more factors:
1. Not enough respondents, so some attribute levels were either picked 0% of the time, or 100% of the time.
2. Deficient (poor) design, caused by too many prohibitions.
3. Trying to estimate interaction effects among attributes involved in a prohibition.
If you continue to have questions or problems on this, you could send your project files to technical support (firstname.lastname@example.org) and ask them to look at it.