Also Check out this:
http://www.php.net/manual/en/json.installation.php
HTH

This one did the trick ! Thank you so much :)
now to check this out some more.

for future reference for other users with the same problem:
I fixed the issue with


sudo apt-get install php5-json && sudo /etc/init.d ...

wxop wrote: BTW you might also turn the debug on , in the PMD config file, and see if you get any error printed on screen or in the apache error_log

This did show an error:

Fatal error: Call to undefined function json_decode() in /var/www/inc/lib/pmd_root_api_client.php on line 483

that was the complete error_log, i went into a little nerd rage and reinstalled the complete apache/php/sql stack :)
apache2ctl restart didn't do anthing.

I reinstalled (from git this time instead of the zip file), The first time i got the notification "pmd has been installed, please refresh if ...

adding "Allow from all" did not make a difference, the logs also don't show any clue as of what is happening...

Access.log

192.168.1.113 - - [03/Apr/2014:18:47:46 +0200] "GET / HTTP/1.1" 302 243 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.154 ...

I'm trying to get the PMD interface to work on my linux machine, but it's not working.
I used to get a forbidden message, i fixed that but now i only get a white empty screen....

my .htaccess file:

#####################################################
# BEGIN Controller
<IfModule mod_rewrite.c ...