got error when it opens two different file for extern editor

I am trying open .xls file external editor (excel) and it opens first file fine but then when I open second file I get following error

"WinSCP was not able to determine application that was started to open the file. WinSCP Cannot watch for changes in the file and thus it will not upload the changed file back

One possible reason for the problem is that the file was opened is some already running application

If you want to use the application to open files from WinSCP, Consider configuring it as an external editor

Note that the file will remain in temporary folder."

Please help
Thanks

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