Issue:
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After upgrading your Client machine to Windows 8 OS with Office 2010 + SP1. Excel Sheets from SharePoint sites which open on Client Application for editing, open up as 'Read-Only' Mode.

 

Workaround:
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On the Windows 8 Client machine create the below registry key. This is Client machine issue, NOT SharePoint Server related. 

1. Open regedit.exe

2. Naviagate to following key:

[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\14.0\Common\Internet]
"FSSHTTPOff" = dword:0000001

3. Click Edit Menu -> New -> DWORD with name of "FSSHTTPOff" (without quotes)

4. Click on "FSSHTTPOff" and enter value of 1

5. Close any Office Applications and browser sessions

6. Try to repro the problem