HP Officejet scanner: "Failed to open a session on the device"

I have an HP Officejet 4500 G510n-z printer/scanner, connected to the local wifi. I'm trying to scan from my macbook running OS X Yosemite (10.10.5), but when I try to do so, I get the error message "Failed to open a session on the device". This has worked before, with the same computer and scanner, so I'm trying to figure out what has changed, and how to fix it. In Preview, the File menu has an "Import from Officejet 4500 G510n-z" entry, which suggests that it recognises that this is a scanner. Selecting that menu entry gives the above error message. I get the same result when using the "Open

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