What would a dispute between two bishops look like?
While I realize that much depends on the local church (orthodox, catholic), vs england, italy, germany etc... I'm generally just curious what disputes between churches or even parishes might look like - and how it could involve magi.
Like some other topics I have brought up, this will need to take into consideration the religious beliefs of your players, but...
... a lot of issue which are considered canon now were still being debated back in 1220. The Mallus Malefectorium wasn't published until the 1400's, which means that doctrine on magic, it's use, and existence. It is also a time of theological turmoil preceding the Inquisition. From the catholic culture website
Um, the Mallus Malefictorium was a refutation of the existing belief (which was basically that witches did not exist and that people who thought they had supernatural powers were deceived or mentally ill). After about three years, the Catholic Church declared that the Mallus was doctrinally false, BTW. For example en.wikipedia.org/wiki/Canon_Episcopi contains some French law that was in force in the game period, where priests are meant to force people who think they are witches from their communities, and gives an example of women who are deceived into thinking they are part of the Train of Diana. It notes that believing people can change shape into animals is heresy (Bjornaers regularly break the period-defined True Nature of humans in Ars. This is a known thing, which we just ignore because Fun).
The book had surprisingly little impact in real history. Earlier historians quite liked to talk about it, because it let them slag off Catholics or Protestants (depending on who the writer was), but it's basically a historical curio.
It is easiest if you remember that bishops are "nobles" who control territory. So, many disputes would be territorial ones. At a parish level "is this highly productive field (which contributes a big tithe) in my parish or yours", and a similar thing at large scale at the level of a bishop ("is this town in my diocese or yours?").
Oddities like monasteries would be sources of disputes too. For example, "300 hundred years ago the monks were granted land to build a monastery and in return paid a tithe to the bishop, but the bishopric was combined 200 years ago with its neighbour, but now the pope has re-established the original bishopric --- who gets the tithe?" So, you have two bishops with claims, the monks would rather pay neither, and appealing to the pope is risky, because the pope may well decide that the pope is the appropriate person to receive the tithe.
Whenever a bishop exercises any of his administrative functions in an area that borders or otherwise impacts on another bishop there is a possibility of disputes. For example, say, a baron is excommunicated by a bishop, but the baron's lands lie within two dioceses, and the other bishop supports the baron.
Any of these issues could easily involve magi. Ones that explicitly involve magi might be things like "the covenant has a licence to practise magic without fear in their local diocese --- the magi then go and cause trouble in neighbouring diocese, and claim the protection of their home bishop".