When Moodle exports data to CSV (e.g. course completion reports), it includes column headings in the first row. This is not a problem, except when the first column of the CSV file is an ID field. In this case, the CSV file starts like this:

ID, col2, col3, col4

Unfortunately, Microsoft Excel can't handle CSV files where the first two characters are 'ID'. It looks like Microsoft use a bat-**** insane method of detecting file formats. If you open a CSV in Excel and the first two characters are 'ID', Excel decides it's not a CSV, it's actually a SYLK file. It tries to open it as a SYLK file, and fails. Because it's not a SYLK file, it's a CSV.

Now, this is obviously Excel's fault, not Moodle's. Microsoft do have a knowledgebase article for this issue, but their helpful answer is "we won't fix this, just make sure your CSV file doesn't start with 'ID'."

The official answer to this is to enclose 'ID' in double quotes. A simple solution in Moodle would be to enclose everything in the column header row in "double quotes".

Microsoft Knowledgebase article on this issue on Excel 2003 (we can confirm the issue is also present in 2007 and 2010): http://support.microsoft.com/kb/323626

Helpful blog post on the issue: http://efeamadasun.com/post/35734306022/excel-has-detected-that-csv-is-a-sylk-file-but-cannot