If you're losing coolant, that's a bad sign. Is the sludge a light color or a dark color? If it's dark, like NuclearHateLizard said, that's oil run too long.

If it's light, like a cafe latte, it's usually moisture. In that case moisture rises as the engine heats so emulsified oil collecting on the underside of the fill cap or potentially on the dipstick is expected.

If you're DIY inclined, there's a kit you can pick up at most autopart stores that will tell you if your headgaskets have failed and are allowing combustion gasses into the cooling system (go on YT and search for 'Combustion Leak Detector' and watch a couple of videos.

If you're not getting combustion gasses in the cooling system, it could be failing radiator or oil cooler (depends on the car, I don't know VWs). If that's the case it could be a 'simple' fix and not a big job like a head(gasket).

Beyond that, there's no quick fix and if you watch your coolant levels and your temp gauge like a hawk you might be able to get by for a bit until it fails catastrophically. My advice is have AAA and stay inside the free tow radius.