Question: I'd like to run a pptp vpn server on a raspberry pi in my home network behind a fritz.box.

It should work like this:

Android - internet - fritz - raspberry-pptp - internet

Client can connect, but no internet access.

1723 and GRE are open on fritz and should go to raspberry.

How do I have to iptable to make Android get internet via raspberry?

Answer: 
You fried your pi because the **V+ is wired to PWR+** so you just sent 12V to your pi. There is not much information about this driver board but there is a thread at the raspberry pi forum and someone even made a schematic of the circuit. You can read more info here: <https://www.raspberrypi.org/forums/viewtopic.php?t=241060>  

Appart of V+ being wired to PWR+ there is also an important fact:

> 
> The two enables, ENA and ENB, if left disconnected, default to enabled
> high with 10K pull-ups to Vcc. If you need to control them, the
> IR2104S shutdown inputs enable at 3V rising and 0.8V falling.
> 
> 
> The four inputs, IN1A [the first input for bridge A], IN2A, IN1B, and
> IN2B, have 10K pull-downs, and 1.25V thresholds. With this, they are
> 3.3V, 5V, and 12V logic compatible, as long as low is less than the threshold.
> 
> 
> 

I am sorry for your pi but I hope this helps in the future.
I can't see anything obviously wrong with the schematic.

Likely causes.

* connecting more than 3V3 to a GPIO
* connecting more than 3V3 to a 3V3 pin
* connecting more than 5V to a 5V pin