Can you use the ping command as the Checkmk site user? If not, check the capabilities of /bin/ping and ensure that your filesystem allows extended attributes.
$ getcap /bin/ping
/bin/ping cap_net_raw=ep
$ xattr -l /bin/ping
security.capability:
0000 01 00 00 02 00 20 00 00 00 00 00 00 00 00 00 00 ..... ..........
0010 00 00 00 00 ....