MKRZero I2C Problem on the Slave using Wire-Library

After working successfully with the MKRZero and I2C a long time, I now have the problem that apparently Wire.onReceive on the slave is no longer triggered.

I verified the problem with a playground of two connected MKRZero and the simple code examples taken from the Wire library. The result is exactly the same, Master sends, but it stays with a NACK.

I tried both IDE 1.8.9 and 1.8.10.

What changed?

Code.pdf (36.3 KB)

I'm missing the wiring diagram!