"z-axis homing issue, may have an external disturbance"

Hi. My K2 has been happily printing for a few weeks now, but all of a sudden started having print quality issues first, and then complete bed adhesion failures, which appear to be linked to incorrect z-homing.

After trying to run a z-axis calibration, I got an error "z-axis homing issue, may have an external disturbance", which I now get every time I try to print with a calibration cycle included.
Creality's documentation says "check z-axis sensor", but I have no clue how to check it (I don't even see a sensor, I understand it's a strain gauge inside the extruder assembly, but I don't see anything that looks like one in the assembly).

Has anyone seen this error? Any clear troubleshooting steps? Do I have to replace the sensor, and if so, how?

Any guidance will be appreciated.