The Start/End Date and Time fields directly under the event title and description are the start and end date/time for the singular event, and not for the series. For most repeating events the start and end date will be the same but the start and end time will be different. You may have events that have a different start and end date, for example I've seen a week long scout camp on the calendar that spans 6 days, but again, this is the start and end date of a single event. For repeating events that occur on a given day, you don't want to change the end date, it should be left the same as the start date. Select the This event repeats checkbox (which I presume you have already done since you are already seeing the repeating frequency information) and then add the End Date information in the Repeats Until/End Date fields.
I'm guessing there might be a limitation to how long a given event can span, but you should find that if you don't try to use the left hand side End date and time fields for repeating events you won't have the error you are getting.