A quick google search indicates that the error can be related to several things in Excel: "The error code
0x800A03EC (or -2146827284) means NAME_NOT_FOUND; in other words, you've asked for something, and Excel can't find it."
Could it be that what you are trying to write to Excel does not conform to the size or format of the sheet? I see that you populate the DownloadedFiles-variable with other variables - this might be part of the issue? Could be that the list is longer than what fits into the sheet or the format isn't accepted?