You will get this error if you try to connect(2) a socket(2) to a host, and a router between you and that host believes the host is unreachable. It would then send ICMP_DEST_UNREACH, which would find its way back to you eventually.

You would also see this instantly if for example your local machine knew it had no route to the destination host.

lib/main.php:944: Notice: PageInfo: Cannot find action page

lib/main.php:839: Notice: PageInfo: Unknown action