This message may appear (instead of “Solver encountered an error value…”) if the Interval Global Solver encounters an arithmetic operation or function that it cannot evaluate for the current values of the decision variables.  Recall that the Interval Global Solver evaluates Excel formulas over intervals such as [1, 2] as well as real numbers.  In the course of seeking a solution, the Solver may have to evaluate a formula that (for example) involves division by an interval containing zero, or the square root of an interval containing negative values, which yield errors.  If you receive this message, try adding constraints, or adjusting the right hand sides of existing constraints to eliminate the problem.  For example, if you have trouble with a constraint such as $A$1 >= 0, try a constraint such as $A$1 >= 0.0001 instead.