I have found the problem:
root@hst-105-124:~# dpkg -l
...
ri libssl0.9.8 0.9.8o-4squeeze14 SSL shared libraries
ii libssl1.0.0 1.0.1e-2 SSL shared libraries
...
And my installed version of Nginx seems to depend on the wrong libssl (0.9.8):
root@hst-105-124:~# dpkg --remove libssl0.9.8
...
nginx-full depends on libssl0.9.8 (>= 0.9.8m-1)
...
root@hst-105-124:~# dpkg -l
...
ri libssl0.9.8 0.9.8o-4squeeze14 SSL shared libraries
ii libssl1.0.0 1.0.1e-2 SSL shared libraries
...
And my installed version of Nginx seems to depend on the wrong libssl (0.9.8):
root@hst-105-124:~# dpkg --remove libssl0.9.8
...
nginx-full depends on libssl0.9.8 (>= 0.9.8m-1)
...