This one had me stumped for a while. A Canon LiDE 110 scanner on a Windows 7 64 bit machine; worked fine while logged in as one user but not as another.
When trying to scan, to any location or with any output format, it popped up with a message saying “Unknown error. Code =19”.
Both users had admin rights, both had new profiles on the machine. The only difference was that the user it didn’t work for had redirected folders. Changing the scan location didn’t make any difference, it still gave the same error.
After some digging in the registry, the solution was to edit the following key
HKEY_CURRENT_USER\Software\Canon\MP Navigator EX\4.0\CanoScan LiDE 110\Folders\ScantempFolder
to point the location to C:\temp. The software was scanning to a temporary folder before outputting to the path the user had chosen, and the location it was trying to use was redirected and inaccessible.