Live css module won't save my css
The error is generated by the following code in the module

  if (file_munge_filename($sanitized_url, 'css less') != $sanitized_url) {
    echo drupal_json_encode(array(
      'result' =&gt; 'failure',
      'filename' =&gt; $href,
      'msg' =&gt; 'The url used contains a sub-filextension which poses a security threat. Saving not allowed.'
    ));


This checks passes the sanitized url through the file_munge_filename function. The function checks for all the extensions in the file adds an underscore after the extension to make it secure. you can read the link for more information.

So I presume that there is some issue with the css file you are uploading. there might be more than 1 extension in the file name such as style.abc.css.