Hi Klaus,
Does it still say "Connected" under /administrator/index.php?option=com_htprotect&sec=connect?
If the connection was lost for some reason, the integrity check can no longer be delivered.
Sounds like the site became unreachable first, and it wasn’t triggered by the remote integrity check that ran before that?
You could try reconnecting. Disconnect in the Joomla backend at the URL mentioned, then reconnect using a new connection code from the dashboard via "Add Website".
Cheers
Pascal