Complex sentence: Janssen left the band in the summer of 1990, disillusioned about their new label's neglect in properly promoting the band abroad, and one of the most promising Dutch bands of the era fell apart.
Split into simpler sentences: Janssen left the band in the summer of 1990, disillusioned about their new label's neglect in properly promoting the band abroad. With that, one of the most promising Dutch bands of the era fell apart.