triac controlled voltage preregulator (mains)
The problem as mentioned in the question has been solved. As I have suspected Tri2 did not behave as wanted in quadrant 3 and 4. I replaced the original triac TIC206D with a sensitive gate triac Z0405NF. Now the triggering of the circuit operates as required.

For those who are interested, a short explanation of the circuit.
The zener diodes limit the incomming mains up to a value of 66 V peak.
This voltage starts to charge C1 through R30, P1 and BR2. The opto coupler connected to BR2 acts as a variable resistor thereby able to influence the time it takes to charge C1. 

As soon as the breakdown voltage of BR1 is surpassed Tri1 triggers. At the same time Tri2 triggers ( voltage over R32) discharging C1 below the breakdown voltage of BR1. There after the story repeats until the voltage accros the zeners becomes to low to charge C1 high enough to trigger BR1.

For more info read the information in the Application note of SGS THOMSON.

Time to move in making the system function as intended.