This is most likely due to the speed of the connection, the flash site isn't very usable for people on connections slower the DSL, and if there was a drop in connection at any point, no matter how short, it could cause error messages. There are actually many things that could cause an error message, one is if 'http 1.1' is not checked in their browser. Below is a suggested resolution from Microsoft if the error 501 is experienced.
To resolve this issue: 1. In Internet Explorer, click Internet Options on the Tools menu.
2. In the Internet Options dialog box, click the Advanced tab.
3. Click to clear the HTTP 1.1 check box, and then click OK.
4. Test to determine if the issue is resolved. If the issue is resolved, do not complete the remaining steps. If the issue is not resolved, go to the next step.
5. In Internet Explorer, click Internet Options on the Tools menu.
6. In the Internet Options dialog box, click the Advanced tab.
7. Click to clear the Show friendly HTTP error messages check box.
8. Try to view a Web page to reproduce the error message and to receive a new, more descriptive error message. This new error message may be similar to the following:
HTTP version not supported by program name.
Note the name of the program in this new error message.
9. The program name that you noted in step 8 may be the cause of this issue. Either remove this program, or contact the manufacturer of the program to inquire about the availability of a fix for this issue.