Lotus support answer

HCL Notes/Domino 8.5 Forum (includes Notes Traveler)

Subject:

Since this occurs for you only with 8.5.2 FP4 and not with 8.5.2 FP1 I checked the existing customer records about this problem and found customers which experience this issue also with 8.5.2FP1. The range is starting from 8.5.0 and goes up to 8.5.3.

According to our SPR JKAE83U78M this only occurs, when the Excel file is open and a temporary file exists.
I searched on the Microsoft Website and found the following:
http://support.microsoft.com/kb/814068
According to that the temporary file is removed when the file is successfully saved. So it might be just coincidence that it started with FP4 at your site when the users have temorary files from Excel around.
You can try if the behaviour does not occur if you save the changes to the Excel-Sheet before using the send-to function. I doubt that it helps since the only workaround described is to close Excel and then attach the file manually to the eMail.

The fix for this is expected to be implemented with 8.5.4 which will be released in Q4 2012.

So let the users save the excel file, before using Send To > Email

Feedback response number WEBB8TMGGT created by ~Zelda Chujipyterobu on 04/23/2012