Quick update: there was a small piece of plastic jamming one of the film advance gears. That plastic piece came from the shutter speed adjustment ring, which was intended for better grips but apparently easy to detach itself after the glue fails.
In general, the Werra Matic is very well designed and constructed. Very modular: the rangefinder/viewfinder is one single block, and the meter is another single block. So once the top is off, it is very clear what is going on. The engineers were clever yet very disciplined.
To remove the top, you need some special tools to loosen the screws on the strap eyelid. And pull out the top cover just a tiny bit to unscrew the viewfinder eye piece. Once the top is off (gently), there are a few loose pieces you need to keep track of. There is one clear plastic piece that sits on top of the lightmeter unit. And there are a stack of washers along the shutter release shaft.