You also have to connect ground (GND) for signal reverence level, shielding, putting both devices on same ground level, etc...
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I don't know what you're doing with pins 22 and 23 but those aren't used for the Serial Connection. Those are pins 8 and 10, the TX and RX. Also, you are connected the TX of one to the RX of the other, correct? They have to cross. The receiving for one device has to connect to the transmit of the other.
Or you could just use the USB connection like it was intended to be. 
It's going to be more reliable because it is shielded. Where the GPIO headers are not and are subject to possible weak connections or your header wires popping off.
@ryan780 RX and TX should be correct as there is data coming throught and the setup worked fine like that. It is as if the signal is noisy so I guess @Ewald_Ikemann might be on to something. Just so that I do not do something stupid ... Connecting the ground on the pin 2 on the einsy board to pin 3 on the pi (the pins marked in black on the diagram above) without connecting the 5V pin (red) would do the trick right?!
Pin 3 on the Pi board is the I2C SDA line. You have to learn that there are 2 standards that the pins are numbered by. The board order and the BCM order. You are using neither of them and it makes it next to impossible to help you. The ground pin near the top of the GPIO header is pin 6.
And I am just telling you how the serial bus on the RPi works. If you connect TX to TX and RX to RX nothing is going to work. But hey, you aren't having any problems at all, right? Oh wait.....
@ryan780 Ok I checked the official pin numbers and I want to connect pin 6 on the raspberry board to pin 4 on the einsy board (the ones marked black on the diagram) and as you said if RX was connected to RX and TX to TX nothing would work, but as I said there is real readable data coming throught it, is just broken up by garbage characters and missing pieces. So those pins must be correctly connected
Nothing like trying things out I guess, so I just connected the ground as said and lo and behold I got myself a beautiful serial connection. No garbage characters, no resends just pure data. So the USB cable will probably not come out again until it´s time to update the firmware. Thanks @ryan780 for your detailed answers, much appreciated, and @Ewald_Ikemann for pointing to the missing ground connection. I hope that I have now a stable pi without untervoltage powered by a safe powersupply with a nice direct UART connection und a working octoprint update system....
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The low voltage error shows it obviously isn't getting enough power. The pi doesn't randomly throw that error just for fun
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