There is a bug in the logic when following a URL to a resource that the browser downloads as an attachment and you are not already authenticated. When you follow the URL you are presented with the login screen. After you enter your credentials successfully, the file downloads but you are still on the login screen page. If you then click Login again, it will redirect to /sakai-access and you'll receive a 404.
1. Upload a file to a site that the browser would normally download as an attachment, ie a zip file.
2. Copy the URL to that file.
4. Paste the URL into the browser address bar and press Enter.
5. You'll be taken to the login screen. Enter your credentials and clock Login. The file will be downloaded.
6. Click 'Login again'. You can see a brief redirect to /access/login and then it will dump you at /sakai-access with a 404.
This can also be reproduced by following steps 1-5 but then entering your credentials incorrectly. You will be taken to /access/login (ie the full URL is stripped). This time enter them correctly. You'll get the file. Now do step 6, ie click Login again, and you get the same error.