Thank you for your comments!
@Tuerer: It works in the TPS example project
@Jacky: I don’t know exactly what you mean by “Consume Input” but I checked my Blueprints and the Character Blueprint seems to be the only one which uses the InputAxis-Event(s) … and each of the events is only used once.
@ThompsonN13: I disabled/enabled the “Inherit pitch” and “use control rotation” options for my camera boom without any effect. I tucked a print string behind the event and it always seems to fire with an output of 0.0 … even if I don’t move the mouse at all.
It is weird that the yaw works but the pitch doesn’t. :-/