Hi Giannis.
do not give up that early.
After some discussion as well with an contact at Opel it was recommended from them to change the chain tensioner and use another Oil type.
The background is, that after many complaints for the the earlier produced machine types they invented and developed another improved chain tensioner.
This is what i will go first and see next, If this was not chnged so far at your car i might be the same...