i got "HTTP Error" when uploading imgs under 1.4.4.5 with flash uploader. it's ok when i'm using http to upload.
and meanwhile it's all ok under my another 1.4.4.4 installation.

Probably permissions. Your server error log should tell something. If not it might be server security.

Note the flash uploader will not be part of the coming 1.4.5 feature release anymore.