Hey Torsten,
This problem is caused by your system plugin "IFTA Onesignal". It is blindly using the document API in Joomla without checking the type of the document. This same issue will exist in your Joomla articles rss feed too.
To fix this, I had to edit the file /plugins/system/ifta_onesignal/ifta_onesignal.php and at line 36 - 40 replaced the following codes,
[gist]
// Im Adminbereich nicht verwenden
// auch nur wenn SSL aktiv ist
if ($app->isAdmin() || !$u->isSSL()) {
return true;
}
[/gist]
With,
[gist]
// Im Adminbereich nicht verwenden
// auch nur wenn SSL aktiv ist
if ($app->isAdmin() || !$u->isSSL() || $doc->getType() != 'html') {
return true;
}
[/gist]
You should contact the developer of the plugin to fix their plugin