I solved the problem myself like this:
In Medieval Latin they use frequently an abstractum pro concreto: so here servitium = servum And so we have: he devoted himself as a servant of a church
An other possibility is this: you can see servitium as an accusative to stress the meaning that is already in the verb mancipare. This is possible here, cause they are in the same semantic field.
Then we have: he devoted himself id est his service of ...
The problem of the ac is also solved: Sometimes they put ac with a relative in Medieval Latin, without changing the meaning. So you don't have to translate this ac. qui ac = qui
Problem solved, now I am gonna find an other one,
Cordially and thanks to all who helped,