You may only withdraw from an academic assessment or examination if there are reasonable grounds to do so. If you are unable to take an assessment or examination due to illness, you must submit to the examination board an original medical certificate issued by a doctor.
You can either submit a certificate of incapacity for work (German title: Arbeitsunfähigkeitsbescheinigung) or a filled out form for medical withdrawal from an examination.
The relevant contact at the examination office and the postal address are available here.
Candidates who do not attend an academic assessment or examination on the set date without providing evidence of reasonable grounds will be deemed to have failed the assessment or examination. Candidates who attempt to influence the result of an assessment or examination by deception or by the use of unauthorized aids shall receive a fail grade for that assessment or examination.