Complex sentence: The club suffered a devastating blow in the 1990-91 season when they missed an opportunity for promotion to the 2.Bundesliga when they were sanctioned for the use of an ineligible player.
Split into simpler sentences: The club suffered a devastating blow in the 1990--91 season when they missed an opportunity for promotion to the 2. Fußball-Bundesliga when they were sanctioned for the use of an ineligible player.