Users cannot download purchased files. No file download initiated.

I am running the latest up to date version of Drupal 6 and Ubercart setup to sell file downloads. It's been working fine for the past 24 months.

Problem 1
Recently several customers have emailed me as they cannot download their files. I created a test account and purchased a file to test.

When going to the 'files' section of 'my account' purchased files are visible. Clicking on one of these loops back to the same page. No file download is initiated.

When I try the same thing logged in an administrator, everything works as expected and the file download begins.

I have checked permissions and authenticated users have 'download files' permissions etc. (I haven't changed the permissions recently).

Problem 2
In addition, the link which users are sent via email taking them to their purchased file does not work. A message is displayed stating that "this is not a valid download link".

I have rolled back to the previous version of ubercart and this has temporarily resolved the first problem. The issue with email links still remains.

Please advise.