At the mean time, can you please navigate to the corresponding category page and then upload your files from there?
For example, if you want to upload K750 theme:
1. Go to Downloads > Sony Ericsson Themes > Free K750i Themes / K750 Themes
2. Then, click the upload button
We apologize for the inconvenience. A detail announcement regarding the new upload system will be made once the upload link under My Account is fix. Thank you.