Created attachment 107933 [details] The certificate warning Akonadi should not try to sync with external servers (in my case the DAV resource) when the device is connected to a captive portal. If you configured to use a TLS secured connection, you'll receive many (!) certificate warnings if the captive portal web site is also available on a TLS connection and if your requests will be redirected to the captive portal website. Maybe you can use NetworkManager to determine if you are in an captive portal: $ nmcli networking connectivity check
We tried using NetworkManager to detect whether we are behind a captive protal, unfortunately it sometimes had false positives. Maybe things have improved since then.