Created attachment 60096 [details]
original file, resaved (damaged) file, screenshot of cond formula after and before saving.

Problem description: 
when RELOADED a good Excel file, cell reference in conditional receive sheet reference, and formula get damaged (sheet reference is forbidden in excel conditional format)
If saved with new added sheet reference, the file is no more correct for MS Excel, and loose conditional formatting.  
The xls file is still readable (and correctly interpreted) by LO

The "sheet reference adding", is not present it the file is and ODS format (it happens to be addend only if XLS)

The sheet reference si added also if copying a sheet form an xls file, to a new one.


Steps to reproduce:
create a such file:
1. create a file xls with conditional format
2. save it, and close the file
3. reopen the xls file an look at the conditional formatting formula (the sheet reference has been added)
4. save the file and it no longer will be usable in Ms Excel.
5. open in Ms Excel, and the conditional format is gone

Notice:

in the FIRST file save, the file  is correct, and Ms Excel reads it correctly.

WHEN REloaded once more and resaved, cell reference in conditional receive sheet reference, and get damaged ( because sheet reference is forbidden in excel conditional format). 


Current behavior:
LO shouldn’t add any sheet reference in the conditional format formula.

Expected behavior:
The conditional format formula should not be changed

Platform (if different from the browser): 
              
Browser: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:11.0) Gecko/20100101 Firefox/11.0