I suspect that this may be a data related issue. But regardless, it is an issue with the tagset. The tagset is generating something Excel does not understand.
In addition to posting the sas code and log, I would look at the XML file ExcelXP tagset is generating. I know it has extension of xls but it is really XML. Use an application to view it which does not attempt to parse the XML, for example notepad. Don't use excel.
Using the XML parsing error info you present you may be able to track down the error. If not then I suggest contacting SAS tech support and in particular the ExcelXp tagset team. Or if you can find someone with expertise in Excel XML standard, they may be able to help you.
I have encountered the missing whitespace error where ExcelXP had used a SAS datavalue which is longer than what Excel expects. The specific example I encountered was trying to create excel tab names which were longer than what excel allows. The tagset apparently failed to check the length of the tab names and excel did not see what it expected.
Also if I remember correctly, Excel generates an error log which may be more helpful than the screen error message,