To filter the right information within a database I do this:

Field A is compared with field B within the relationshipAB: Only records will be shown when A machtes B

Field B is a calc.field from another File, that itself uses a realtionship to a third file for the calculation.

This shows no result. When I replace the calculation field by a "normal" field and when I enter some possible values everything works fine.

Help needed. thanks!

If the calculation cannot be indexed, then it cannot be used as the right-hand side of a relationship, only the left. Your relationships graph will designate this as a sideways "T", as it does when the left key field is a global, meaning that data cannot flow that direction.