Thanks benmalan. Tried, that, didn't work.
Here is what works:
1. The external keyboard works fine when connected to other gear
2. SO4 tracks receives the MIDI signal, as the indicator lights up when a keay is pressed.
3. A selected VST is properly connected in SO4 - the onscreen virtual keyboard triggers sounds from MIDI fine
4. The track's Input settings are fine
5. The keyboard is set up as instrument under SO4 Options. I even tried to delete the keyboard as instrument and reinstalled it. I made sure the configuration is the same as when it still worked.
Interestingly, after reinstalling the keyboard as instrument, the string sound disappeared. Now there is now sound at all when a key is pressed on the external keyboard - but there is when a key is pressed on the virtual keyboard.