Is a doctor sinful if he is forced to undress patients down to their undergarments only (while keeping their inner clothes on) during his work in a hospital in Germany, and what should he do about this matter during Ramadan?

It is not permissible for a doctor to expose a patient's 'awrah (private parts) except in cases of extreme necessity, and when there is no female doctor available for a woman. Necessity is to be judged by its extent. If undressing patients leads to the exposure of their 'awrah, then it is not permissible except out of necessity and to the extent required. If you are compelled to expose 'awrah without necessity, then you must leave this job. This ruling applies equally to Ramadan and other months. Undressing patients and touching them does not invalidate the fast by itself, unless, for example, pre-seminal fluid is emitted.