According to laws of the newly formed Austrian Republic every Habsburg might become a citizen as well as obtaining residency if only they would officially abandon any claims to govern. Wilhelm chose not to abandon those claims and de jure lived in Vienna illegally.
The laws of the newly established Austrian Republic allowed Habsburgs to become citizens and residents, but only if they formally gave up their right to govern. However, Wilhelm refused to do so and thus existed in a state of unofficial residency in Vienna.