My answer is very similar to the one from @biscoop, but I am going to elaborate a bit as it may apply to the question or to other people.
I had a .xls that was an extraction from one of our webapps. The Excel connection would not work (error message: "no tables or views could be loaded"). As a side note, when opening the file, there would be a warning stating that the file was from an online source and that the content needed activation.
I tried to save the same file as an .xlsx and it worked. I tried to save the same file with another name as an .xls and it worked too. So as a last test I only opened the source .xls file, clicking save and the connection worked.
Short answer: just try and see if opening the file and saving does the trick.