Can athlete’s foot cause any other, more serious, diseases?

Other infections. Athletes foot, a fungal infection, can cause cracks and fissures in the skin, which can serve as an entry point for a bacterial skin infection leading to cellulitis. Additionally, the fungal infection from athletes foot can also lead to a fungal infection of the toenails.
Rarely, but yes. Athlete's foot can cause blistering or cracks in the skin. If these open wounds become infected that could lead to a more serious problem.