SW2017 SP2 won't install - Previous Toolbox detected. Select installation method.

When trying to update our installation, we get this message:

The "Change" and "Install now" icons are greyed out, how do I select installation method?

Background: Our toolbox is installed on a server (mapped drive, not UNC). I have downloaded the install files, and run the upgrade from this. Tried on three different PC's with the same result.

SolidworksAdministration