Indeed the STVA motor connections on this bike are dodgy. When you do the soldering repair, you need to be quite careful to ensure that the solder doesn't short to the aluminum motor end-cap, and that you retained those insulating plastic shims inside the motor to also prevent shorting. I added tiny heat shrink tubing to the wires I added to my STVA PCB board traces, just for extra insulation, just in case.
If you got C28 code all the time on the bike, you may have had a dead STVA motor (that soldering the contacts won't fix) but that isn't super likely. My C28 code was intermittent, and only happened at high RPM on my bike, and after the soldering fix it has never returned.
So I would make absolutely certain that your repair worked, and that the motor wasn't already shot to begin with. It's worth checking the 4-prong plug that connects to the motor as well, make sure the connector and contacts are clean and not worn.
And make sure the main harness isn't damaged. Trace the 4-prong connector back to the ECU plugs just to be sure there is no shorting and that all 4 wires are making it all the way back.
Once you've got it together, check that the secondary butterfly valves open fully when you turn on the ignition and killswitch. Push the secondary butterflies gently closed when the bike is off if you need to.
Make sure you didn't mix up the two TPS sensor plugs too, just in case. It happens all the time.
Hope this helps.