Also, to get the old toolbox uninstalled, I had to reboot into safe mode. Otherwise the uninstaller just hung. Then booted back into regular mode, removed all traces of the existing printer, then reinstalled the new software and it works great.
I had previously tried running the MF Toolbox as an admin (compatibility, all users, as someone else here mentioned) but the problem according to procmon was that mf network scan wasn't able to fire off MF Toolbox. Tried UAC, etc, nothing. That's when it occurred to me it wasn't mf toolbox for me, it was the network scan.
Some incompatibility, when some other software felt the need to update a boatload of .NET assemblies or virtual engines or redistributables, and it's broken. Now, the toolbox doesn't know what scanner it's talking to. Probably because some WSDL XML got screwed with and the RPCs are all messed up...
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