Yep, I had the "same" with my TT 2001...
In my case, one of the breathing hoses had loosened and dragged on the ground while driving. When breathing the hose sucked up snow/water from the road like a kid using a straw a hot summer day (it was during the winter of course with lots of rain and snow). Water in oil is not a good mix causing all slipping discs in the Haldex coupling to fail.
My case is unusual, the breathing pipe should not fall off like that, but never changeing the oil is not good at all and will eventually have the same effect.
I would go for oil and filter change (once or twice) to see if it is enough. With luck, the clutch discs are not toasted yet. With bad luck you need to change them.
It is possible for a mechanic to open the Haldex and change them, but finding a complete unit might be cheaper.
For me it was easier/cheaper to buy a rear axle with Haldex attached from a crashed car. I got one off a MkIV R32 as they are the same as Mk1 TT 2001.