According to Microsoft:
A SYLK file is a text file that begins with “ID” or “ID_xxxx”, where xxxx is a text string. The first record of a SYLK file is the ID_Number record. When Excel identifies this text at the beginning of a text file, it interprets the file as a SYLK file. Excel tries to convert the file from the SYLK format, but cannot do so because there are no valid SYLK codes after the “ID” characters. Because Excel cannot convert the file, you receive the error message.
I got the error message:
‘SYLK: File format is not valid’
when trying to open a CSV file.
Apparently this is a known issue in Microsoft products.
Read more on KB323626