> Is there anybody discussing that there may be a logic behind this decision? E.g. Erdogan pressures German publicly to gain advantange in some other discussions with Germany. Or maybe he wants to gain a stronger following among his people and they like when he starts a fight with a stronger country, and he needs that stronger following because his political opposition is quite strong. Something like that?

Yes, that in fact, minus the strong opposition. His power is not that big, but the conjecture allows him to keep on: The three strong opposition parties cannot form an alliance against his. See my other comments.

I'm not sure how closely you follow the Turkish news, but this is an everyday level of stupidity by Turkish government standards.

He really is dumb enough and believes his own hype enough to think that Germany will back down if he threatens certain things, such as ending cooperation on Syrian refugees.