Typically when there is a display error of this type on either the Asset Rebalancing Report or on the Asset Management Tab in the Fund Module, the actual value attempting to be displayed is a meaningless percentage. It will usually occur when the asset balance across all the accounts involved in rebalancing is close to zero, but the assets need to be moved.
One balance is a very large positive and another balance is a very large negative. Since the denominator in the calculation of the percentage is very small, and possibly 0, the percentage calculates to be a very strange number.
Ignore the percentage and examine the actual balances in the accounts and if the proposed changes make sense, continue to process the rebalancing. If you rerun the report or look at the screen after the ensuing journals are created and posted, you might find no error.