Answer: political ambitions of the church (the biggest institution of that time). Not all religions are institutionalized but Christian religion was and is institutionalized. Each institution is composed of people (imperfect with their flaws). These flaws became increasingly evident in 14th century.
Explanation: indications of decadence: 1) kings and emperors got more and more in power conflicts with papacy: (a) holy empire empiror Heinrich IV (so-called "Canossa"), (b) French king Philip IV Le Bel, (c) holy roman empire king/emperor Louis the Bavarian (first half of 14th century), then papacy tried to suppress voices of liberty (philosophers) within the church itself and became increasingly dogmatic which led to movement of "conciliarism" (creation of councils that were supposed to substitute the authority of the pope.....15th century).
