We recommend, that you please contact the respective health insurance provider directly.
In principle, the statutory health insurance providers will collect the dues for the students’ health and long-term nursing care insurance. Mandatory health insurance is required through § 5 para. 1 no. 9 Fünftes Buch Sozialgesetzbuch (volume V of the social insurance code) as long as students are not insured through a parent or spouse (family insurance) or were exempted from the health insurance requirement. If you possess a valid European Health Insurance Card (EHIC), you are not legally required to purchase additional health insurance coverage in Germany.
You can be covered by student health insurance until your age 30. Thereafter you have the possibility to enrol in a voluntary health insurance programme.
Doctoral students can not, in principle, have student insurance. Unless another employment circumstance does not require mandatory health insurance (e.g. at a department chair), the student has a voluntary insurance option.
If you as a student do not meet the obligations of the statutory health insurance system, then you can not be enrolled or re-registered (§ 254 volume V of the social insurance code). As a consequence, you will not gain admission to the university after the enrolment period expires. Furthermore, if the grace period for admission expires, you will be exmatriculated.