The software isn't recognized by Yosemite. How can I fix this?

check for the driver in the new os
add the scanner in the control panel section add a device

Try to set up the custom paper size from the Print dialog box in another application and then choose it in your application that was printing blank pages.

Best I can do: Will your Mac run OS 10 10 Yosemite ZDNet

Hi
If you can't scan then it sounds as if your computer doesn't have the appropriate driver installed for scanning. There are a lot of drivers and other downloads here:
Printers
including a scanner driver for MAC OS 10.3 to 10.7. Since Yosemite is 10.10, I don't know if this would work for you, but I think it your most promising solution.
It may be worth disconnecting the printer and connecting it directly to the computer. If it scans and prints when directly connected then it is likely that the print server is causing the problem and if as you say there is no Mac software for it you may be in a no-win situation. The fact that you can print through it however does mean there is hope, because I have found this page which is described as manual installation of the server on Mac OS here
FAQ ID 347 How to install Print Server manually on MAC OS
but the OS is 10.6.4 so, I'm not sure.
Not as helpful as I had hoped but perhaps you can get something from this screed

you need the proper OSX-System Disk according to your Mac and startup from there
e.g. for Yosemite:
How to Clean Install OS Yosemite

x2
Business Plan Pro 15th Edition 32bit load that. and find your 32bit data.

It should either be on the back of the box or on the back of the cd case.