Open http://my.domain.com/vl/foobar
It should display : 'ERROR 404 from OpenNab'. Some web browsers (e.g. Internet Explorer might need some changes in configuration options to display the actual error message)
If not, you have an issue with Apache configuration (ErrorDocument feature in .htaccess is not available).
You must sort it out before going further.

Open http://my.domain.com/vl/tests/
You should see a green status with all tests passed.
If not, you have a PHP issue. OpenNab code does not work on your PHP configuration.
If you get errors with either PHP 4.4.x or 5.2.x, ask on Nabaztag help forum http://sourceforge.net/forum/forum.php?forum_id=654766
If you get errors with another PHP version, this is normal. You must upgrade your PHP install.

Open http://my.domain.com/vl/bc.jsp?v=0.0.0.10&m=xx:xx:xx:xx:xx:xx&l=00:00:00:00:00:00&p=00:00:00:00:00:00&h=4
You should get a weird text page starting with word 'amber'.
If you have nothing or a PHP error, proceed as previously : check your PHP version and eventually ask for support.
Note that this call will create a burrow for a dummy bunny : http://my.domain.com/vl/burrows/xxxxxxxxxxxx

Useful OpenNab links :
Api demo : http://my.domain.com/vl/api_demo/
Configs folder : http://my.domain.com/vl/config/
Main config : http://my.domain.com/vl/config/opennab.ini
Logs : http://my.domain.com/vl/logs/
Burrows : http://my.domain.com/vl/burrows/
TTS tests : http://my.domain.com/vl/tests/interactive_test_tts.php
Plugin system tests : http://my.domain.com/vl/tests/index2.php
Nabaztag calls :
http://my.domain.com/vl/locate.php?sn=xxxxxxxxxxxx&h=4
http://my.domain.com/vl/p4.php?sn=xxxxxxxxxxxx&v=65808&st=1&sd=0&tc=0&h=4
http://my.domain.com/vl/p4.php?sn=xxxxxxxxxxxx&v=65808&st=1&sd=0&tc=7fffffff&h=4
http://my.domain.com/vl/record.php?sn=xxxxxxxxxxxx&v=65808&h=4
http://my.domain.com/vl/rfid.php?sn=xxxxxxxxxxxx&v=65808&h=4&t=d00218bd6e5693fd