This is an urgent issue (clients can't access their paid media).
After upgrading to the last version of Membership (from version 18.104.22.168) my protected mp3 downloads started working incorrectly in IE and Chrome.
In Chrome: the download is started but when whole file downloaded the browser says “server error” and file is not saved to the disk.
In IE8 - when trying to save target on the link it thinks for a long time (maybe a 30 sec or minute) then shows error message "Unable to download <filename>.mp3 from <sitename>. Unable to open the Internet site. The requested site is either unavailable or cannot be found. Please try again later."
Tried to disable Membership – everything starts working just fine. So it's definitely plugin issue.