The only time I really saw checks used was when I was a child ~30-35 years ago and my parents used them. I did once cash a check from an elderly relative, but that was very unusual and only happened once. I didn't even know it was still possible to do that, my reaction was more like if someone had handed me a stack of punch cards to run on my computer.

There hasn't been anything an average person used checks for in the last decades in Germany. Except a few elderly people, nobody uses checks and there are no rebates via checks at all.

Granny can always give you cash or just send it directly to you account in the same way.

As it turned out, my bank rejected both because they were made out to [middle name] [surname] rather than [firstname] [surname]. Ironically the former is unique (probably) whereas they had another customer with the latter.