Excel Error when opening CSV File: File was detected as SYLK File!

When opening a normal CSV-File with Microsoft Excel, a strange problem occurs.

First, I get the following error message:

The file you are trying to open is in a different format than 
specified by the file extension. Verify that the file is not 
corrupted and is from a trusted source before opening the file. 
Do you want to open the file now?

Yes | No | Help

After clicking on "Yes", the next error occurs:

Cannot read record 1.
Continue reporting each error?

Yes | No | Cancel

I click on "Yes" again, and I get the third error message:

Excel has detected that 'file.csv' is a SYLK file, but cannot load 
it. Either the file has errors or it is not a SYLK file format. 
Click OK to try to open the file in a different format.

OK | Cancel

Only after clicking "OK" in this third error message, my file is opened and ironically, it is displayed correctly.

As far as I can judge, there are no other problems with the file. Additionally, OpenOffice can display the contents properly. So what is it?

Unfortunately, I have to pass this CSV spreadsheet on to a customer and I would not like that every time he gets this error messages. Can someone help me with that issue?