Update:
- Create new circle: Contacts app → Circles
- Add a new user to the circle
- Both users are getting errors in the Activity app
Temporary solution:
vim /etc/crontab
* * * * * root mysql -u root nextcloud -e "DELETE FROM oc_activity WHERE app = 'circles';"
* * * * * root sed -i '/image not found\: image\:png webroot\: serverroot/d' REPLACE_ME_nextcloud_log_file