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PCA9306: Very strange behaviour on SCL2

Other Parts Discussed in Thread: PCA9554, PCA9306

I really need your help with this issue I am facing right now.

You find the schematic attached. The I²C Bus consists of the PCA9306, two PCA9554 and an EEPROM.

Basically, the problem is that SCL2 and SDA2 are not pulled to 5V like they supposed to be and therefore I2C communication does not work.
And it becomes even more weird that I found out modifying VREF1 has an Impact on that behaviour.
You find the scope shots for the different VREF1 powering up attached. Please note the cursors indicating the time when the voltage drops and the level of the I2C signal to which it drops.

Does anyone have a clue what is going on here and what I can do to make the PCA9306 operate the way it should?

Thank you very much for your support!