SAMD11 I2C (SERCOM0) never leaves IDLE state

I finally managed to get I²C working on the ATSAMD11D14A, and it turned out there were two separate issues.

1. Even though PA08 and PA09 can be mapped to SERCOM0, they cannot be used for I²C Master

The SAMD11 SERCOM pinmux allows many pins to be connected to SERCOM0, but I²C Master mode only works on SERCOM PAD0 and PAD1.

So I had to switch to a valid PAD0/PAD1 pin pair.

The only available pins left for me were:

#define SMARTAGI2CSDAPINMUX  PINMUXPA04DSERCOM0PAD0
#define SMARTAGI2CSCLPINMUX  PINMUXPA07CSERCOM0PAD1

After switching to PA04 (SDA) and PA07 (SCL), the SERCOM finally generated clock pulses.

2. A required clock was not enabled

The SAMD11 uses the GCLKSERCOMSLOW clock source for SMBus/I²C time-out features.
In my case, I had to explicitly enable

SMARTAGI2CSERCOM->I2CM.CTRLA.bit.LOWTOUTEN = 1;

Edit: Added Images of the I2C Bus actually probing an reciving back the RXNACK