I’ve unboxed the Syntakt, plugged in headphones and played a few of the preset patterns. All I can hear is the FX track.
I’ve upgraded to 1.0.1, same thing.
I’ve done a factory reset, same thing.
Anybody else ever had a similar issue with an Elektron device?
EDIT: I suspect it doesn’t like headsets and I will need to try with headphones later.
Yes, TRRS connections could well create problems
I’d rebrand this as a ‘hack’
Well, the connector is TRS, so I still find it strange, but maybe it’s the 1/4" adapter. I was just wondering if this had happened to others on similar devices.
I’m probably being stupid but isn’t this a routing issue? I route just the wet effects to the main outs when using overbridge and sometimes forget afterwards and end up listening to the effects only and scratching my head a few days later.
I think the same thing. However, the OP said this happened after he unboxed it so I assumed the unboxed unit was new. If it was a B-stock unit, it’s possible it was returned with manipulated audio routing settings before it was purchased again 
I did a hard reset. That should have taken care of any routing issue, no?
The connector on your headset might lead to cancelling out the mono signal which is why you might only hear the stereo effects…
I’m not sure. Just to be safe, go to your audio routing options and make sure all your tracks are routed to main. It’s possible that only your effects are routed to main.
Yes I would have thought so but as @algorithmtoo says worth double checking the audio routing settings in the main menu.
I will try various things in a few hours and post back.
Luckily I had a laptop with me and used USB audio to route it to the headphones via the PC.
So… It was a dodgy adapter. Not sure what was happening in there because I didn’t get a mono signal in one ear, but some stereo wet signal.
Thank you to all of you for trying to help! 
Glad it’s all sorted out. Now you can really have fun with your new gear!