Important change to the enrolment regulations (§ 38 Abs. 5 Landeshochschulgesetz):
Previously, PhD students were required to be enrolled until submission of the dissertation. With the new regulations, you are required to be enrolled until the date of your defense.
This means, unless you defense takes place until May 13, you have to re-register for the summer term 2026.
If you don't re-register, you will have to re-enroll for the time of the defense.
Generally exempt from above regulations are PhD students that have been freed from the enrolment as per §38 Abs. 5 Landeshochschulgesetz (requires employment >=50% at Ulm University).