this error may have several reasons:
1) the easiest: did you add your nagios server IP in the allowed_host line of your nrpe.cfg file ?
2) did you compile your nrpe and check_nrpe with the -ssl-enabled option ? if yes, you shouldn't use the -n option (both executables have to use the same encryption).
3) if you don't really mind about security, re-compile both executables without ssl; it might work better
4) And last one I can see: (It happened to me). The nrpe executable is not able to find the required librairies. Check it with this command: "ldd nrpe"; all the librairies should be present ...
If one of them is not found, you'll have to implement it, or recompile or add it to the LD_LIBRARY_PATH variable.
I hope this helped you