idProduct=5740 seems to show that the original STLink FW (inside U19) has been erased and replaced by a custom application (i.e. not using ST-Link upgrade) which now seems to do only the USB VCP you see (this could have been done with another STLINK connected on CN12 ST-Link factory SWD port).
Btw, this setup does not allow ST-Link upgrade as it is now not anymore an ST-Link.
I think there is no way to restore original FW (at least on my side, I don't know).
You somewhat 'bricked' the ST-Link part of your board.
If you did not find any way to restore embedded ST-Link functionality and don't want to buy another DK1 board, ultimate solution might be to use an external STLINK-V3SET by forcing U19 in reset (SB18) and connecting external device to relevant signals (SWCLK, SWDIO, SWO, NRST, UART4, Ground, etc..) from existing point (e.g. SB17, 19, 20, 22 and R77, R72).
Regards.
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