I just finished my first quad a few days ago, flew a few packs and... crashed. I found it on wet grass. Two arms broken. I then plugged it on my bench with just the two engines which remained attached.

When I plug the battery I can hear all the tones, produced by both engines. However in Betaflight motors tab, only one of the two motors spins. The other one has an error rate of 100% (denoted as E: 100.00%, from bidirectional dshot). Moreover, if I leave the quad plugged for too long, the 0% error motor makes a sound as usual, the 100% does not.

Please help me troubleshoot this problem.

You've likely killed the MOSFETs on the ESC. Try swapping over the two good motors with the other two to see if the good motors then stop working on the suspect ESCs and if the non-working motors then start working on the ESCs that the original working working motors came from. If the answer to that is yes then you will need a new ESC.

I did a connectivity test between the pads and it looks fine. And if a MOSFET was killed the corresponding motor wouldn't make a sound on startup, right?

Anyway, I've just ordered a new ESC, but will also check the old one using the method you described above. Thanks.

EDIT: I've just tried to run the motors via esc-configurator.com - and they both worked. I then went back to Betaflight and they both work there now too...
Maybe I fixed it by pushing ESC's harness plug, though I thought it was all the way in already.

I guess I'll have a spare ESC.

I misunderstood your first post. You stated "When I plug the battery I can hear all the tones, produced by both engines." which suggested that only the two good motors were making the ESC startup tones. After re-reading your post I think I understand that the other two motors were ripped off with the arms so you can only test with the two remaining motors which are still connected to the ESC.

Are you getting all 5 startup tones from both of the motors? If you are only getting the first 3 tones from one of the motors then it means there is no communication between the ESC and the FC on the signal wire for that particular motor. That could be because of a bad connection on the FC-to-ESC wiring harness. Try re-seating both connectors by unplugging and re-plugging them. And while they are out, use a magnifying glass to check inside the sockets for any bent or damaged pins.

Haha, I've edited my message above to avoid spam, but it looks like we came to the same conclusion. Thanks for your help!

Also, to clarify, I'm sure I was getting all 5 tones from both engines. I guess I'll need to work on the way I write these posts to make them more clear.

have a good look at the link between esc and fc.
i crashed my 5" and had 100% comms loss on one motor.
capacitor and buzzer had piled into the ribbon cable between the esc and fc, knocking it loose.