The string for textgroup views is not allowed for translation because of its text format

I'm currently using Drupal 7.15 with all modules up to date, including views 7.x-3.3

When I translate a header text for a view, first the translation is accepted. But whenever I edit the view to change any parameter, the translation is destroyed with the following message : The string views:my_view:my_view_block:header:views:area:content for textgroup views is not allowed for translation because of its text format..

I confirm that I enabled all Translatable text formats in string tabs in Multilingual settings. How could I stop that painful behaviour ?