Missed registration deadline that was not announced for second exam of a course, how should I proceed?
As proposed by @Sumyrda - remember Monica, I post my comment as a possible solution.
As a lecturer at a German university, I suggest the following: the informal appraoch is via the lecturer. If s/he does not register you for your exam, try a formal approach via 'Prüfungsausschuss' or the 'Prüfungsamt'. I can only speak for a few institution I worked for (from rather small to bigger universities): the processes are quite different and thus are your option to register for tests, exams or written essays, etc. Since COVID19, universities have opened processes here and there for exceptions, especially concerning third tries on exams and registration processes. However, these are regulated on a institutional, often even on a chair-basis.