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I came across this today when you open a new Microsoft Excel 2010 worksheet, the new sheet is not shown on the taskbar, only the main Microsoft Excel icon is shown on the taskbar.
This is because the Excel hook is not injected into any process because the seamless engine component (Sehook20.dll) responsible for the fix fails to load into the user session. Sehook20 fails to load because active accessibility hook is disabled in the Seamless Flags settings. Seamless flags are often used when troubleshooting problematic display issues.
I got round it by adding the following registry key.
Get out the following article for more details.