Just got home from the flats.
After some serious testing my conclusion is this.
The tippet material i used on the previous trip from where my problems started was the issue.
Can't pinpoint the date of purchase and/or manufacturing but do know that this was the problem.
with the same material;
fresh leader/fresh knot > no problem
fresh leader/fresh knot, in the water for 30 minutes and leaving it in the sun resulted in immediate breaking of the leader to tippet knot, breaking right after the knot and leaving the knot on the leader.
don't worry, didnt test it on a fish, used a piece of heavy rock instead, gradually applying pressure was no problem, whem mimicing a strip-strike it broke every time.
Also, tried the uv knot sense stuff, useless in my opinion, just leaves a big klump on the leader, might be usefull for minor repairs tough. for waders
Furthermore, using mud helped for the situations where the fish where really spooky, thanks Paul, for this tip, had never tought of using it on the flats, will never leave home without it!
And Greame; will try a different brand/type of hook, heard some good feedback on the Daichi X-point hooks.