Two choices; Apply to Germany and find a way to prove residence or apply to a different country that doesn’t require proof of residence. Other people have also offered their help and advice.
Someone else already advised that application can be made from anywhere except Thailand. Why apply to Germany which, unlike other countries, requires proof of legal residence? Being a German citizen surely means the applicant possesses a National Identity Card? Assuming so, why not state temporarily lodging with parents, relatives or friends etc? Take a photo with a recognisable background, the applicant holding a newspaper with todays date?
Consulates can (and do) state whatever garbage suits them. It is a case of what is reasonable in real life situations that is the determining factor when a customer requests a chargeback. A citizen has permanent right of abode in his own country. Common sense must apply. As already advised, why not apply in a country that only requires temporary physical presence during the application process?
Very clever. However, rather than attack myself only you should be addressing your acidic question to the group as a whole. I always advise that if any application is cancelled or rejected without reason being stated the applicant should request a chargeback by their bank/credit card company. If the consulate can’t (or won’t) supply an acceptable reason, or fail to respond within 30 days the applicant will get their money back. But you would already have known that wouldn’t you?