click here is the link for the latest Scangear driver.
I used to have a CanoscanD1250U2F and it was always a bit tricky to set up. If I remember correctly you have to install the driver before you connect the scanner to the PC.
Googling Iscapallowed shows that this is a TWAIN driver problem so I would uninstall what you already have (well at least the Scangear CS-U and the Toolbox) unplug the scanner, install the latest drivers, plug basck in and try again.
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