masel

Try a modified process.php look here: http://www.wpdownloadmanager.com/support/topic/download-file-size-0kb/page/3/#post-8347
Try disabling more plugins, one after another.

I created another simplified process.php for 2.5.0 hope this helps some people.

overwrite to /wp-content/plugins/download-manager

watch there for fix a http://www.wpdownloadmanager.com/support/topic/download-file-size-0kb/

did you update all of your plugins? the fix is for 2.4.9 only
deactivate one plugin after the other and try downloading