./letsencrypt-auto does not succeed

I tried to run ./letsencrypt-auto on a newly installed debian 7.9 server and I get the following error messages:

Updating letsencrypt and virtual environment dependencies…
Could not find platform independent libraries
Could not find platform dependent libraries <exec_prefix>
Consider setting $PYTHONHOME to [:<exec_prefix>]
Fatal Python error: Py_Initialize: Unable to get the locale encoding
LookupError: no codec search functions registered: can’t find encoding

In the following you can find the outputs of python from the command line:
$ which python
$ python
Python 2.7.3 (default, Mar 13 2014, 11:03:55)
[GCC 4.7.2] on linux2
Type “help”, “copyright”, “credits” or “license” for more information.
$ echo $PATH


Thx for your help and time

This could help.