When open the attached document, then save it, an exception "System.InvalidCastException: Unable to cast object of type ' ’ to type ' ’ " was thrown out.
Thanks for reporting this problem to us.
While using the latest version of Aspose.Words i.e. 15.8.0, we managed to reproduce this issue on our end. We have logged this issue in our bug tracking system. The ID of this issue is WORDSNET-12379. Your thread has also been linked to this issue and you will be notified as soon as it is resolved. Sorry for the inconvenience.