From other forums, I have learned that it is due to the MS Update and that a good fix is to simply delete the file MSForms.exd from any Temp subfolder in the user's profile. For instance:
C:\Users\[user.name]\AppData\Local\Temp\Excel8.0\MSForms.exd
C:\Users\[user.name]\AppData\Local\Temp\VBE\MSForms.exd
C:\Users\[user.name]\AppData\Local\Temp\Word8.0\MSForms.exd
Of course the application (Excel, Word...) must be closed in order to delete this file.