I guess you have connected the Zynthian’s power connector to your Windows PC’s USB port? It is likely that there is insufficient power available to drive the Zynthian. It may have been on the edge and something just pushed it over. How are you powering the Zynthian?
I have a Pi power supply connected through the special power/data Y cable that has been working without problem for several months. I replaced the Y connector, USB cable and moved to a different USB port and the problem did not go away. Everything else seems to be working (can add remove chains and it passes the “audio test”). I have not tried the DIN midi as it is physically difficult. Have just gone back to the Nov. release and the problem remains. Unfortunately, I just discovered that at some point system restore on my PC got disabled (nothing I did AFAIK) so I can’t go back to a previous version to see if it was some other software load that caused the problem. So blocked for the moment.
I can confirm this is not working currently. It seems to be a regression. g_midi driver is not being loaded and hence the OS does not know to bind its OTG interface as a MIDI device. Manually loading g_midi after boot makes it work, e.g. modprobe g_midi. I can’t see why the module is not being loaded.