Okay, I'll init it LOW.

It was now clear to me.

Checking this out now...

You're right - now it works fine.

Actually, I noticed that all PORTBs are driven HIGH for one second when I power-up the circuit. I initilize them to LOW but this happens after the register setting.

I don't think it would make sense to set the PORTBs LOW before the registers are set, does it?