Error in Roundcube


this error belongs to Roundcube. When I start typing an e-mail address in the to-field, then the following error appears (German):
“Ein Fehler ist aufgetreten! Serverfehler! (error)”
In English: “An error occured! Server error! (error)”

When I click on “contacts”, the website isn’t reachable (HTTP error 500). The address is

It is possible that the source of this error has something to do that I updated the Contacts app of Nextcloud manually to version 3.5.1 inside Nextcloud.

Have you tried rolling back the update?