Is it possible to de-register from exams?

Yes. Until  3 days before the regarding examination, the cancellation via the self-service LSF portal is possible. 

The exact date can also be found on the Examination Office's website under "important dates". Thereafter, examination registration is binding (i.e. it cannot be undone)! If an exam is scheduled to take place outside the examination period, you cannot deregister. 

In the event of an unexcused absence during a registered examination, the exam performance will be considered "not sufficient" (5.0). Reasons for withdrawing from a registered exam must be reported promptly, in writing, and with credible evidence to the Examination Board. Such a reason can be, for example, illness. In such an instance, you should first send a written notice to the examination office listing the exams you cannot attend due to the illness. Additionally, immediate proof of illness by a medical professional is required (forms are available directly in the Examination Office or on their homepage).