[ 1689.287067] usb 1-1-port2: attempt power cycle
[ 1689.944132] usb 1-1.2: new low-speed USB device number 35 using xhci_hcd
[ 1689.976945] usb 1-1.2: device descriptor read/8, error -32
[ 1690.126960] usb 1-1.2: device descriptor read/8, error -32
[ 1690.344155] usb 1-1.2: new low-speed USB device number 36 using xhci_hcd
[ 1690.377003] usb 1-1.2: device descriptor read/8, error -32
[ 1690.527052] usb 1-1.2: device descriptor read/8, error -32
[ 1690.644438] usb 1-1-port2: unable to enumerate USB device
Hmm, looks like for whatever reason it fails to get detected properly. It looks like you’re using Pi 4 4GB version, I tried using the same kernel version and Midiboy gets detected properly every time it is connected.
This may indicate an issue either with the cable (you could try a shorter cable, or one with a ferrite bead(s) near the end(s)), or some soldering issue on the board. Make sure the solder joints are all well done. Also make sure the components appear roughly in the same places and same direction (for polarized components only) as in the picture on https://blokas.io/midiboy/assembly/4/
Even though your computer detects it right, some other USB implementations might struggle communicating with the same device due to noises induced somewhere, or small mistakes in the schematic.