I had TWO DKs as described in the title randomly fail on me in the same way, months apart. Additionally, I had an earlier rev DK, 2.1.0 (NRF5340 is NOT the IMCU on this version), fail in the same way. Very frustrating and no forums posts or guides helped at all. Seems to be a common problem which is often left unresolved or the solution is "buy a new DK". Found a better solution that may help some.
Symptoms:
All switches are in the correct position.
Known good USB data cable plugged into IMCU_USB connector
LED5 flashes rapidly
jlinkexe / nrfutil / lsusb cannot detect the probe
Pressing / holding "RESET" button while power cycling makes no difference in behavior.
Investigation:
Digging into the schematic (V3.0.2 here), I noticed the IF_OFF net controls the switch U24 that connects incoming USB power (~5v) to VBUS_IMCU (VBUS / A5 pin on NRF5340) on the interface MCU. The IMCU needs 5v on this pin.
If the default / NRF_only switch is in the default position, IF_OFF should be high if USB_DETECT is high. I verified USB_DETECT was high on my board. I'm pretty sure IF_OFF was 3.01v, as measured at the circled pin here on SW6:
Measuring both sides of SB48 showed a delta: one side measures 3.5V, and the other measures 5.2V. Clearly the switch is not closed although I believe in this state it should be.
Solution:
The solution was then to short SB48. Doing so resulted in totally normal behavior as before.
BONUS SOLUTION:
Without digging through the schematic for my V2.1.0 DK, I found SB48 visually and tried shorting it. Again, the board worked totally normally after that. I'm assuming the architecture is similar even though it does not use a NRF5340 as the IMCU.
Diagnosis:
Given that IF_OFF was high, and based on the DS for TCK106AG, VIN/VOUT should be connected when CTRL is HIGH, I believe the switch U24 simply failed for some reason.
If this helps anyone, please let me know and post below. I'm wondering whether this a defect or something I'm doing (cheap USB hub?) that might cause these issues.