I was about posting here to get help to set this up, but then it dawned on me. But i think it may help others too, so i post it anyway.
I started to write two words, but then i thought: "Wait a minute... could it be, that...". And yes, sure enough it was (again) that the trigger level was at zero. The thing that got me confused was, that it triggered well with other settings of the pulse trigger, but it did not so with negative pulse trigger when between two values.
It may be a bit embarrassing, but if it helps others, i am fine with that
The followong may only apply to siglent SDS800 series:
I like to mention, that after a scope restart, trigger levels of channels that were not active in that particular trigger, are set to zero if activated.