This message appears if you are solving a mixed-integer quadratic programming problem, and the Hessian of the quadratic objective is not strictly positive definite.  When this is true, the QP subproblems solved as part of the Branch & Bound process may not have unique optimal solutions, and hence the Branch & Bound search cannot guarantee an exhaustive evaluation of all possible integer solutions.  If at least one integer feasible solution has been found by the time the search is terminated, the best such solution is returned by the Solver, and this message appears.