In reply to MarkAstley:
It's a long time since I've touched a dt swiss hub, so take this with a pinch of salt...
Had a quick look at the service manual here https://www.dtswiss.com/en/support/manuals/
Looks like there are two bearings in the hub shell, as listed in the parts table, and at least one more in the freehub body. This is a fairly common arrangement across lots of hub designs.
If the wheel can freewheel freely (sorry) then chances are the hub shell bearings are more or less ok, and there's an issue with the freehub. If you have the tools I'd start by removing the cassette and checking nothing has gotten wrapped round the freehub/axle. If it's still stiff, follow the dismantling instructions till you get to picture 4 in section 2.3, i.e. you've removed the freehub body and part of the ratchet mechanism. If you look in the freehub body, there should be a cartridge bearing (the red bit in the picture). If you poke your finger into the middle you should be able to spin it freely, if not the bearing has seized.
There's no instructions for stripping down the freehub further, so chances are dt swiss want you to buy a new freehub body. You could see if you can punch the bearing out to replace it (more than likely a part you can get from a decent bearing supplier), though there may well not be sufficient access to get a drift onto the back of the bearing.
If you have a lot of time to spare at the moment, you could try prying out the seal and cleaning and re-greasing it. They never last long after this if they've already gotten to the point where it's seized but you might get a bit more use from it.
Edit: seems you've figured it out, sorry typed too slow
Post edited at 15:05