I had the rear brk. lock up as well. It was the "stop bolt" under the brk. lever. It had lossened up and applied the brk. Turns the disc blue and wore out the pads before I caught it. replaced the pads, but still using the "blue" disc.
The chain and the tires are kind of substandard and don't last long. I replaced the chain at 1,000, Km and went one tooth bigger on the counter shaft sprocket[12T to a 13T] at that time.
The filter should be first rinsed in solvent[gas], then water and left to dry, then an app. of foam filter oil. Don't over-oil it.
I've had no other problems, currently at 8,000 Km.
Would like to find a better rear shock though, the stock one is awful.