How to configure the l10n server address ?

The module will use the localize.drupal.org server by default, everything will be autoconfigured after fetching the main server file, http://ftp.drupal.org/files/translations/l10n_server.xml

New servers can be defined by the module info file or by additional modules.

Check notes in the code for how to add a new server from module.info file http://drupalcode.org/viewvc/drupal/contributions/modules/l10n_update/l1...

Other modules can define new servers too by implementing hook_l10n_update_servers(). See http://github.com/jreyero/atrium_features/blob/master/atrium_translate/a...

The module's documentation will be updated with to answer this question. You have (currently) two options:
* Write a module that implements hook_l10n_servers() With this you can set the translation server name and url for all projects on the site.
* Add 'l10n_server' and 'l10n_url' definitions to the .info file of a project to override the server settings for this project only.

please read the code...

for the info file:
l10 server=..
l10 url=..
l10 path=..

so no _

 * - Localization server to be used for this project. Defaults to http://localize.drupal.org
13	 *   l10n server = localize.drupal.org
14	 *   (This should be enough if the server url, the one below, is defined somewhere else)
15	 *
16	 * - Metadata information for the localization server
17	 *   l10n url = http://ftp.drupal.org/files/translations/l10n_server.xml
18	 *   (We can fetch *all* the information we need from this single url)
19	 * 
20	 * - Translation file URL template, will be used to build the file url to download
21	 *   l10n path = http://ftp.drupal.org/files/translations/%core/%project/%project-%release.%language.po
22	 *   (Alternatively you can use the %filename variable that will default to '%project-%release.%language.po')
23	 */

Yep, you are right. In the current release you will find a README.txt file with this documented.

Just in case anyone else hits this issue while searching for the error message and finds this issue like me, the adress of the localization server for the Drupal.org server needs to be: https://localize.drupal.org/ including the trailing slash (/). Otherwise you will get the error message about it being invalid.