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After upgrading from K2 4.6.11 to K2 4.7, the K2 Designer File Attachment Control intermittently causes the SmartForms Application Pool to stop. This happens when selecting a file and clicking OK in the file browse box.
An unhandled Microsoft .NET Framework exception occurred in w3wp.exe 
APICommunicationException was unhandled --> An unhandled exception of type 'SourceCode.Hosting.Exceptions.APICommunicationException' occurred in SourceCode.Forms.dll. Additional Information: Not Connected
- Set ConnectionPooling to false in web.config for both Designer and Runtime.
- Add an attachment control onto a view.
- Save and publish the view.
- Run the view and load a file with the attachment control, refreshing and repeating until the error occurs.
- Ensure you have K2 4.7 installed.
- Install the K2 4.7 February 2017 Cumulative Update.
- Get the K2 4.7 February 2017 Cumulative Update FP3 from Regional Support.
- Install the K2 4.7 February 2017 Cumulative Update FP3 to apply the fix.
- Clear the Browser Cache.