My trailer is attached to my roadbike with a weber coupling system. How much weight can I pull safely without breaking my frame?
There's tongue weight (the weight pushing your rear axle down from the attachment point) and towing weight (weight pulling the rear axle backwards).
Tongue weight is limited by your rear tire, but most should be able to handle another 50kg assuming the rider is not overweight. Most trailers are designed to be balanced such that there's minimal tongue weight although braking forces can change this.
Towing weight is dependent on your rear axle, rear dropout, and frame. It's also dependent on you the rider accelerating and decelerating smoothly. A mountain bike or touring bike will have the strongest frame.
People regularly tow loads of 80kg (200lb) of children and groceries. People with utility trailers can tow canoes and loads of several hundred kilograms easily. You just have to balance the load on the trailer so that it's over the trailer's axles and putting minimal tongue weight on the bicycle; and accelerating and braking gently to minimize towing loads and strain on your rear dropout. Thus, the trailer itself tends to be the limiting factor
Tl;dr: Your trailer should be list its maximum capacity. Assuming you have a bike with a strong frame and accelerate and decelerate carefully, your trailer is more likely to dictate its limit than the bicycle or mount point.
Tl;dr v2: Weber's website notes its coupler is suitable for loads up to 80kg. The coupler is made of stamped steel so is perhaps more fragile than other designs.
I can't comment on your specific hitch, here's a generalisation.
The normal quoted figure for an automotive trailer or caravan is ~45% weight on each wheel and 10% on the nose.
A bike trailer is likely a single axle - if yous has 3 or 4 wheels then it should stand by itself and noseweight is rough;y half the weight of the drawbar.
I've successfully pulled trailer with two heavy loads.
First was 68 kilos of car tyres, pulled for 20 km: thus
I've also carried 100 kilos of UPS hardware for 6 km. That was right on the limit of comfort. However I know the nose weight was around 9 kilos and each tyre carried 43-44 kilos.
NOTE This assumes your load is just stuff. If you're hauling live people then absolutely stick to the manufacturer's recommended maximum weights.