Hi Dan, hey Zak,
Thanks a lot for the quick response and the clarification! This is a lot of useful insights you delivered, very appreciated.
D’ouh, that insane Lerp value that I put in there didn’t help of course. But even after setting it back to zero the result did not change.
So after looking at Zak’s posting and his set up I removed the “set scrub mode” node completely and also moved the “Set Playhead Time” and “Note On” node into the “Event Tick” controlled section of the blueprint. See pic “GranularSynth_fixed” below:
This did then indeed deliver the expected result, the playback was jumping to the 5 secs mark in the center of the wav file and playing back a short looped section. Funny thing is that I did not even have to add any randomisation math to the playhead position, it seems to slightly randomise already the playhead position then as you can see from the debug read out of the playhead position in the pic below:
&stc=1
Two more dumb questions then 
1.) The playhead position read outs above are matching a playhead position around the 5 sec mark - is it possibly displayed in samples instead of seconds? ( 5 secs ~240k samples @ 48kHz).
2.) Is the “Start Synth” node required at all, because I could not find it in the water stream blueprint and removing it from my set up had no negative effect as well?
Cheers and thanks a lot for the help!
Felix