Hi, the same happened to me this year in the spring, see attached photo. This is after about 85,000 km and 6 years. Astra K is rubbish in terms of quality, my previous Astra H had original exhaust for 9 years (and double the distance, approx. 165,000 km) without problems.
Regarding the repairs, I asked my Opel dealer to fix it, however the original part (ST, 1.6T, B16SHT) for middle silencer is quoted 900 EUR just for parts without labour. For reference, sport exhaust from Fox costs half of that (https://www.fox-sportauspuff.d…ont-silencer/OP044010-VSD). In the end I let them weld it, however I'm aware that this is just temporary solution