Hello,
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 https://mail.domain.de/mail/?_task=addressbook
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.