Felix,
here is a version that works - DiscretizeRegion[] often saves the day!
f = {Indexed[#1, {1}]/(
    Indexed[#1, {1}]^2 + Indexed[#1, {2}]^2 + Indexed[#1, {3}]^2), 
    Indexed[#1, {2}]/(
    Indexed[#1, {1}]^2 + Indexed[#1, {2}]^2 + Indexed[#1, {3}]^2), 
    Indexed[#1, {3}]/(
    Indexed[#1, {1}]^2 + Indexed[#1, {2}]^2 + Indexed[#1, {3}]^2)} &;
planeRegion = DiscretizeRegion@ImplicitRegion[z == 5, {x, y, z}];
tr = TransformedRegion[planeRegion, f];
Region[tr]
