Thanks for your feedback.
I think I found a solution, but I don't know if it is the most elegent one.
On the woody machine which i don't want to upgrade right away, i installed a nrpe-2.0 daemon (compiled it myself). Now the communication works over SSL.
I needed to change the config file (added nrpe_user, nrpe_group) and the commandline of nrpe was changed (from 'nrpe -d /etc/nrpe.cfg' to 'nrpe -c /etc/nrpe.cfg -d').
After having done this. Most things seemed to work.