KB370014: "com.microstrategy.web.objects.WebReportMessageImpl cannot be cast to com.microstrategy.webobjects.RWMessageImpl" when executing prompted documents from MyPage in MicroStrategy Web 10.9 and 10.4 HF6

SYMPTOM:
Users execute a prompted document from the Recents section in My Page in Strategy Web, and run into an error that initially may seem to occur randomly:

com.Strategy.web.objects.WebReportMessageImpl cannot be cast to com.Strategy.webobjects.RWMessageImpl
*JavaScript errors were encountered on this page.* Descriptor with id 2947 not found

1. in Strategy Web, go to Preferences and set My Page as Start Page 
2. go to Web Preferences > Project Defaults > History List and set reports and documents to be added to the History List automatically 
3. execute a prompted document so it appears in the Recents on My Page
4. execute the prompted report used as dataset in the previous document 
5. do not answer the prompts and go back to the Start Page by clicking on the project link (e.g. MicroStratgey Tutorial) in the navigation bar
6. execute the document from the Recents again and notice the error 

CAUSE:
This is an identified defect in Strategy Web 10.9 and 10.4 HF6.
ACTION:
Currently this issue is being reviewed by Strategy for a potential code fix on an upcoming Strategy release. Strategy will continue to investigate and post updates as soon as they are available.
  370014