The process cannot access the file "\\<MACHINE>\Proseries\ProNet21\21Data\ProTaxExport.xml" because it is being used by another process.

ProTaxExport.xml is the file that PS exports with the extracted Client Data for each return.

It looks like reads this extracted data and compiles the Email with the following. 

ProNet21\common\ClientChecklistTemplate.xml (Checklist email template)
ProNet21\common\GlobalText.xml  (The information that goes in the Checklist email)
ProNet21\common\PDFEmailGlobalText.xml  (The information that goes in the Checklist PDF)
ProNet21\common\PDFEmailTemplate.xml (Pdf template)

The file handle to ProTaxExport.xml is not being released before the next checklist is started.