just noticed that the input thresholds for TTL are not compatible with the resitor network on the probe pin. >= 2.0V is TTL high but the resistors source ~ 2.5V compatible with CMOS thresholds. One more resistor and an output pin of the ATTINY should do the trick.

update: The Problem was fixed by a redesign of the probe precharge resistor network and bypassing the resistor connected to Vcc in CMOS mode.