Try following if it works for you -
--Right click on your ".rpt" file Select "Properties"
--Build Action as "Content"
--Copy to Output Directory as "Copy Always"
--Check your path to the .rpt file
Note: if both rpt and aspx page with report viewer are in same folder then no need to give the full path , you can give only filename.rpt in Server.MapPath.
Reference:
c# - A document processed by the JRC engine cannot be opened in the C++ stack - Stack Overflow[
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Hope, it helps :)