After frustrated attempt to report this as a bug in Testlink (http://mantis.testlink.org/view.php?id=3477) I found the solution...
This appears because the connector wasn't enabled in Testlink FCKEditor distribution.
Follow these steps:
- Go to testlink/third_party/fckeditor/editor/filemanager/connectors/php and edit config.php file
- Find $Config['Enabled'] = false; and alter to true
- Find $Config['UserFilesPath'] = '/userfiles/' ; and alter to 'testlink/upload_area'
Please Testlink administrators: many users can not insert images in theirs tests because this error. This should be changed to work without problems, 'cause this version of FCKEditor is part of Testlink distribution
PS: the directory should be writable access