Hi Janes,
I understand that you encounter a problem when opening files in Office Web Apps.
As you said, you have had on issues until today; did you configure/modify the settings for Office 365/IE9/system recently?
Here are some steps you can refer to troubleshoot this problem.
1. Re-run the Desktop Setup for Office 365.
2. Change the Browser to see whether you can open files in Office Web Apps if possible.
3. Have you installed different versions in one computer? If so, you can refer this article to solve the problem: http://support.microsoft.com/kb/2540922
4. Alexandru provided useful information; please also try these steps to see if it will help.
@Alexandru, thanks for your effort and time.
Thanks,
Bourne Zhang