This is really awkward
I seem to have lost my way interpreting the Arduino style connectors on the Cerbuino. It is actually PC1 I’m using (as you already mentioned in your mail)
What I did in Tera Term was E(rase) and U(pload using Xmodem 1k) the Cerb Firmware v0.6.0.glb file.
I use Ms Visual Studio Community 2017 to build and start applications.