we recently had those weird errors – using the flash uploader or the html uploader didn’t improve the situation: the dreaded HTTP error kept on buging us until we realised our error: the wordpress install was protected by a password (apache’s .htaccess) and the solution was lying there as well: the following has to sit in .htaccess in the wp-admin folder fr the time the blog is protected by a password:
AuthType Basic
AuthName share
Satisfy Any
Order deny,allow
Allow from all
<IfModule mod_security.c>
<Files async-upload.php>
SecFilterEngine Off
SecFilterScanPOST Off
</Files>
</IfModule>
* we didn’t monitor what would happen to this htaccess after an automated upgrade