Checked SDA and SCL voltages, and they're ok.
On the EDGE I have now the I2C slave powered by GND and VIN pins of MKR slot.
I then proceeded to test an I2C program on 3 different arduinos: DUE, MKR1010 and EDGE (with empty MKR slots).
The test consisted of:
1 - connect a GY-521 to the I2C
2 - run I2C scanner; results on Monitor
3 - disconnect GY=521 from I2C
4 - run I2C scanner; results on Monitor
Note that with both Due and MKR1010 I could read values from the GY-521 but not with EDGE, even though the GY-521 was connected and found by the I2C scanner subroutine.
I also expected a similar output on each board from the I2C scanner without anything connected to it, but on the MKR it still shows I2C addresses 0x60, 0x68, and, on the EDGE, address 0x22.