Hello Dominik,
First of all, the Manager property is single valued. It is impossible for a suer to have multiple managers. The restriction comes from AD and is not related to Adaxes. If you specify a group in the property, the only way to check its members is using a script. At the same time, All Managers is a virtual property in Adaxes used to search through all the managers in the chain. For example, if the user being processed has a manager, it will be checked first, then if that user has a manager, that user will also be checked and etc. If that is what you need, just change the operator to equals from includes.