The co-owner does not give the keys to the apartment

His actions are illegal. You need to call the police and submit a statement about taking measures against the ex-husband who prevents you from accessing the apartment

In the future, it is necessary to apply to the court with a statement of claim for the elimination of obstacles in the exercise of the right to use the apartment and to oblige your ex-husband not to obstruct your access to the apartment by handing over the duplicate keys to the front door of this apartment.

Article 391 of the Civil Code of Ukraine establishes the right of the property owner to demand the removal of obstacles to his exercise of the right to use and dispose of his property.

The specified rule of material law defines the right of the owner, including a residential premises, house or apartment, to demand any removal of his violated right from any persons in any way that the owner considers acceptable. Determinant for the protection of the right on the basis of this rule of law is the presence of the plaintiff's right to property and the establishment by the court of the presence of obstacles in the owner's use of his property. At the same time, it does not matter who exactly caused the violated right and for what reasons.