Avrdude: stk500_recv(): programmer is not responding ....,,

Hi

since 3 days I have a problem uploading code to an arduino uno. I have tried 3 different boards (uno, nano) , different usb-cables, 2 different pc's running latest Windows 10. However, always the same message.
COM Port seems to be recognized correctly and shows Arduino when plugged to usb.
Before, all worked fine for months ....
Im using arduino ide 1.8.15, then uninstalled it, replaced by 1.8.14, then 1.8.13 ... nothing worked.
I finished using a blank uno board, no pins connected, just simple blink ... nothing
any help would be appreciated, since it starts to be really boring ..

Message is:

avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0xa6
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 2 of 10: not in sync: resp=0xa6
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 3 of 10: not in sync: resp=0xa6
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 4 of 10: not in sync: resp=0xa6
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 5 of 10: not in sync: resp=0xa6
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 6 of 10: not in sync: resp=0xa6
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 7 of 10: not in sync: resp=0xa6
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 8 of 10: not in sync: resp=0xa6
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 9 of 10: not in sync: resp=0xa6
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0xa6

avrdude done. Thank you.

Did you look at the troubleshooting page?

oh yes, I did..
since I changed everything, from board, cable, pc, versions of IDE, I'm asking myself which component at the end could be the problem? Driver problem on Windows 10? A windows 10 update ?
however, windows says driver for com-port is ok

Have you tried the loopback test to test the serial port?

loopback test works fine

after 6 days of try and error and desesperation with this problem, my findings are the following:
to upload a simple blink sketch to my Ardunio UNO, with nothing connected (blank board), I need to go back to Arduino Version 1.8.5 (from 2017, if I got this right). This is the only IDE Version which is working wih my 2 Windows 10 PCs. And even more strange, in the IDE settings, I need to choose Arduion Nano as board. No version above this (from 1.8.6 upwards) is willing to do an upload. Ok, it works, and it works now also with my Arduino on a breadboard Version (with 1.8.5). However, still not the slightest clue what the problem could be – regarding the fact, that before, the actual Version 1.8.14 was working well for months ……

Any chance that you ever tried to burn the boot loader and made a mistake in the process and burned the Nano bootloader to the Uno instead of to the Nano?

thanks sterretje for this input - in fact, I cannot exclude this, have to check. But probably you're right , in fact I burned all my 328p chips all the same way, and put them afterwards in the UNO board or my 'breadboard-Nano' ..
However, the thing that is strange to me is that my 'breadboard-Nano' worked fine for the uploads for months, and all of a sudden, the upload striked - and I didnt change the 328 chip, nor changed any settings in the IDE ... and thats when trouble started, and I started troubleshooting it by using 'simple' configuration: UNO, blink, nothing connected

https://arduino.stackexchange.com/questions/51866/arduino-nano-atmega328p-bootloader-difference

Thanks a lot for this link!
That could finally explain these weird things I encounter , e.g. like while using a Nano , having to choose the Uno board for uploading a sketch, having displayed 32256 Bytes of Memory instead of 30706 by the compiler , however, at the same time, refusing upload of a Sketch bigger than 30706 Bytes. The bootloader Version seems to be the clue..