This error occurs when GN4 is no longer able to find the workflow in memory (typically happens can happen when someone does IISRESET, or the IIS process auto-restarts while the dialog was open).

To correct this error

Wait until IIS reset completes and then retry.

If the IIS reset was not wanted/expected, try to see if there are any errors on the server or with Fiddler.

Maybe something failed on the server while the dialog is open, so IIS was autorestarted?