Why is it incorrect to use the feminine form for professor?
If you have used أستاذة instead of أستاذ in translating the English sentence then it should be correct because in English there is no sign for gender. Probably you should report it as (my answer should be correct).
What if you write: المكتب الاستاذ ? Is it still the professor's office? Or is this never done in any case?
Almost never done in any case. In Genitive (i.e. OF-relationship), if the compound of two nouns is to be defined, the definite article (AL) is added only to the second term. Hence: مكتب الأستاذ.