Nochmal per LED ausprobiert (wollte sichergehen, dass die LCD-Lib nicht irgendwas stört). Verhalten leider immernoch wie oben ;(.

Code:
#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/delay.h>
#include <avr/pgmspace.h>

#define F_CPU 4000000
#define CPU_CLK_64 (F_CPU/64)
#define TIMER_SECOND (CPU_CLK_64/2)




int main( void )
{
sei();

DDRB |= (1 << DDB2);


//lcd_init(LCD_DISP_ON);

	
	TCCR1B |= (1<<CS11)|(1<<CS10);// CPU_CLK/64
        cli();
	//OCR1B=TIMER_SECOND; 
        sei();
	TIMSK |= (1<<OCIE1B); 
	
	
	
 
 
	for(;;)
	{	
	}


}

ISR(TIMER1_COMPB_vect)				//Interruptroutine
{
	if(bit_is_clear(PINB,PB2)) 
	PORTB|=(1<<PB2);
	if(bit_is_set(PINB,PB2)) 
	PORTB&=~(1<<PB2);
		
}