ACE with moderator- warning in output

I'm running an ACE with moderator script, on Mx. When I drop one of the parameters (U), I receive this message-

Mx starting optimization; number of parameters = 7
*** WARNING! ***

I am not sure I have found a solution that satisfies
Kuhn-Tucker conditions for a minimum.
NAG's IFAIL parameter is 6

Looks like I got stuck here. Check the following:
1. The model is correctly specified
2. Starting values are good
3. You are not already at the solution

The error can arise if the Hessian is ill-conditioned

You can try resetting it to an identity matrix and fit
from the solution by putting TH=-n on the OU line
where n is the number of refits that you want to do

If all else fails try putting NAG=30 on the OU line
and examine the file NAGDUMP.OUT and the NAG manual

I'm not sure what the OU line refers to.. any ideas?
The rest of the output works, except for the probability, which says its incalculable.
Thanks!