What is the problem?
OctoPrint and the printer are operating completely normal and reliable but the under voltage warning is showing up. The printer and the PI are both running from a 800W PC Power Supply Unit (as the only user of that power), I doubt that it's a real issue.
What did you already try to solve it?
Ignoring the warning 
Additional information about your setup
Hi @plaetzchen!
Have you connected the PSU to the USB plug or directly to the 5V pin of the GPIO?
Have you the possibility to measure the 5V on the Pi?
Is this the full power 5V rail you connected to, or the 5V stand by rail?
BTW: I have the same configuration, but connected the 5V directly to the GPIO.
Did I say stable? Well... The PI just crashed. So not stable at all. I recently added a Pi Cam to it, maybe that was too much.
I have it connected to the Micro USB in with a soldered adapter coming from one of the square 5V connecters of the PSU.
Hmmm - the red 5V rails should have enough power for a Pi.
So I just restarted the print with the Pi connected to a iPad Power Supply over Micro USB. The under voltage warning is gone. BUT: I started a print and in the middle of no-where the printer stopped and OctoPrint gave a Serial Port error. Maybe the problem is the PSU having issues. I will investigate.
Were you using the always-on rail of the PC supply? It doesn't supply much voltage. And USB length makes a huge difference. If you find a power supply with a built-in cable (rather than simply a USB socket), use it, as it should have power drop built in to the circuitry.
No, I use a socket that is only on when the PSU supplies power to all outlets. I was suspecting the cheap USB cable I use as well but it works flawlessly on the iPad power supply. Maybe the PSU (it's around 5 years old now) has some issues and asks for a replacement.
The 5v rail still may not be rated high enough. There might be a sticker like this that gives the spec. Or maybe the ipad power supply doesn't give the same voltage drop.
I need to fab out my next "pi power supply" today, feeding from 12v (and through a SMPS) to the GPIO pins is much more reliable.