Contact App ist not showing Contacts

Hello, everyone,

i use Nextcloud among other things to synchronize Evolution address books on different Linux computers. This works without any problems so far. But today I wanted to use the webfrontend of the Contact App and found out that no contacts are displayed and there is no addressbook anymore. A new one cannot be created either. The number of contacts is zero. Also in the settings of the Contact App the message “The import is disabled because no address book is available” is displayed. However, if I click on the contact search field in the upper right corner next to the button with Settings, Apps … the existing contacts are displayed. Can anyone help me?

Thanks in advance.