Thanks for your reply, and yes, I did forget to add some crucial information: I'm using a Uno R3 as ISP, the target board is hooked up to its ISP header.
It seems I've accidentally overwritten the schematic with some modifications, but I've attached the PCB layout, maybe that makes it a bit clearer.
The full error message:
Arduino: 1.8.15 (Windows 10), Board: "ATmega328, Yes (UART0), EEPROM retained, 328PB, BOD 2.7V, LTO disabled, External 16 MHz"
avrdude: Version 6.3-20201216
Copyright (c) 2000-2005 Brian Dean,
http://www.bdmicro.com/ Copyright (c) 2007-2014 Joerg Wunsch
System wide configuration file is "C:\Users\1520004\AppData\Local\Arduino15\packages\MiniCore\hardware\avr\2.1.3/avrdude.conf"
Using Port : COM8
Using Programmer : stk500v1
Overriding Baud Rate : 19200
AVR Part : ATmega328PB
Chip Erase delay : 9000 us
PAGEL : PD7
BS2 : PC2
RESET disposition : dedicated
RETRY pulse : SCK
serial program mode : yes
parallel program mode : yes
Timeout : 200
StabDelay : 100
CmdexeDelay : 25
SyncLoops : 32
ByteDelay : 0
PollIndex : 3
PollValue : 0x53
Memory Detail :
Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack
----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
eeprom 65 20 4 0 no 1024 4 0 3600 3600 0xff 0xff
flash 65 6 128 0 yes 32768 128 256 4500 4500 0xff 0xff
lfuse 0 0 0 0 no 1 0 0 4500 4500 0x00 0x00
hfuse 0 0 0 0 no 1 0 0 4500 4500 0x00 0x00
lock 0 0 0 0 no 1 0 0 4500 4500 0x00 0x00
calibration 0 0 0 0 no 1 0 0 0 0 0x00 0x00
signature 0 0 0 0 no 3 0 0 0 0 0x00 0x00
efuse 0 0 0 0 no 1 0 0 4500 4500 0x00 0x00
Programmer Type : STK500
Description : Atmel STK500 Version 1.x firmware
Hardware Version: 2
Firmware Version: 1.18
Topcard : Unknown
Vtarget : 0.0 V
Varef : 0.0 V
Oscillator : Off
SCK period : 0.1 us
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100% 0.02s
avrdude: Device signature = 0x000000 (retrying)
Reading | ################################################## | 100% 0.02s
avrdude: Device signature = 0x000000 (retrying)
Reading | ################################################## | 100% 0.02s
avrdude: Device signature = 0x000000
avrdude: Yikes! Invalid device signature.
Double check connections and try again, or use -F to override this check.
avrdude done. Thank you.
Error while burning bootloader.
Error while burning bootloader.
This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.