Budvar10:
You definitely need not the capacitor (Using Programmer: stk500v1).
It could more in your wiring. Show your setup to us. Do you have minimal circuit for 1284P? Oscillator, power with decoupling and reset.
Thank you a lot. I have managed to burn bootloader once, then tried to upload a sketch using an FTDI converter (not the Arduino UNO anymore). I could not upload the sketch and I could also not burn bootloader afterwards anymore.
I have uploaded the images for the circuit here:
As well as in the attached .pdf
Gescannt_20191017-1431.pdf (1.05 MB)