Please read the debug.log
What does it say right here? **WARNING: No match for value with regex on output 'RTA = ([0-9.]+) ms'.
What is that telling you? Well, look in your nagiostat.conf for the ValueRegexTemplate for most likely ping_rta_pktloss. It states that the regex is RTA = ([0-9.]+) ms, but your output isn't like that is it? Your output is FPING OK - 188.8.131.52 (loss=0%, rta=71.100000 ms)
So rta does not equal RTA. You are using the fping command, but that regex was setup for the ping command, which has a different output.
You have to either modify the ping_rta_pktloss or create another one (I'd create another one) I called mine fping_rta_pktloss and no, I'm not going to tell you exactly what it is, since this should be easy for you to fix.
Now, on to your 2nd debug entry.
What does it say here? **WARNING: Hostname and description didn't match any of the regexes in the config-file.
That means that in your nagiostat.conf file, for all of the many "InsertValue" lines that you have, it could not find any that have a Hostname and description of HOSTNAME=DomainNameServer2
So, check your nagiostat.conf file and make sure you have one line that meets the above hostname and description.