Uncertain forecasts regarding the feed-in by Renewable Energy Sources (RES) can cause overloads in the transmission system. Thus, forecasting errors influence both the decision on redispatch for congestion management and on permissible control reserve activation. In this paper, a probabilistic approach is applied for investigating these two effects. After evaluating probabilistic models of forecasting errors for wind power, probabilistic load flow methods are applied for mapping the domain of credible deviations from forecasts to the domain of permissible control reserve distributions. In a case study of the German transmission grid, the impact of forecasting errors on necessary preventive redispatch and RES curtailment as well as on the permissible space of control reserve activation is analyzed. As part of this, the benefits of dividing the decision on preventive redispatch into a day-ahead and an intraday decision is evaluated.