XF 1.5 Failed to write the attachment thumbnail file ?? (cookie free domain)

[dondomainer]

After create cookie free domain for data, and include on config.php the code

$config['externalDataUrl'] = 'https://static.mydomain.com/data';
$config['externalDataPath'] = 'https://static.mydomain.com/data';

or

$config['externalDataPath'] = 'https://static.mydomain.com/data';
$config['externalDataPath'] = 'https://static.mydomain/data';

i get error when i try to upload any image.

and Server error log say

XenForo_Exception: Failed to write the attachment thumbnail file. - library/XenForo/DataWriter/AttachmentData.php:162

no other plugins are involved



plz, what am I doing wrong??