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.
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:
“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