MightyCore on 1284P - Able to upload sketch only once! - [SOLVED]

[Watcher]

Hi I ve been using the mighty core on home brew Atmega1284P based system for some time now with no issues.
In my latest project however, I came across a very bizarre issue where I can only upload a sketch only once. Later times fail with a "programmer not responding message".
The only way around this is to burn the bootloader again, after which I can again upload a sketch just once!

I am using Arduino As ISP programmer (Mighty Core) as I always did.

Please see below output from :
A. Successful sketch upload

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\John\AppData\Local\Arduino15\packages\MightyCore\hardware\avr\2.1.3/avrdude.conf"

         Using Port                    : COM5
         Using Programmer              : arduino
         Overriding Baud Rate          : 115200
         AVR Part                      : ATmega1284P
         Chip Erase delay              : 55000 us
         PAGEL                         : PD7
         BS2                           : PA0
         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    10   128    0 no       4096    8      0  9000  9000 0xff 0xff
           flash         65    10   256    0 yes    131072  256    512  4500  4500 0xff 0xff
           lock           0     0     0    0 no          1    0      0  9000  9000 0x00 0x00
           lfuse          0     0     0    0 no          1    0      0  9000  9000 0x00 0x00
           hfuse          0     0     0    0 no          1    0      0  9000  9000 0x00 0x00
           efuse          0     0     0    0 no          1    0      0  9000  9000 0x00 0x00
           signature      0     0     0    0 no          3    0      0     0     0 0x00 0x00
           calibration    0     0     0    0 no          1    0      0     0     0 0x00 0x00

         Programmer Type : Arduino
         Description     : Arduino
         Hardware Version: 3
         Firmware Version: 8.0
         Vtarget         : 0.3 V
         Varef           : 0.3 V
         Oscillator      : 28.800 kHz
         SCK period      : 3.3 us

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.00s

avrdude: Device signature = 0x1e9705 (probably m1284p)
avrdude: reading input file "C:\Users\John\AppData\Local\Temp\arduino_build_274511/Soldering_Station.ino.hex"
avrdude: writing flash (31540 bytes):

Writing | ################################################## | 100% 7.91s

avrdude: 31540 bytes of flash written

avrdude done.  Thank you.

Subsequent failed sketch upload

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\John\AppData\Local\Arduino15\packages\MightyCore\hardware\avr\2.1.3/avrdude.conf"

         Using Port                    : COM5
         Using Programmer              : arduino
         Overriding Baud Rate          : 115200
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x4c
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 2 of 10: not in sync: resp=0x4c
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 3 of 10: not in sync: resp=0x4c
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 4 of 10: not in sync: resp=0x4c
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 5 of 10: not in sync: resp=0x4c
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 6 of 10: not in sync: resp=0x4c
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 7 of 10: not in sync: resp=0x4c
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 8 of 10: not in sync: resp=0x4c
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 9 of 10: not in sync: resp=0x4c
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0x4c

avrdude done.  Thank you.

Problem uploading to board.  See https://support.arduino.cc/hc/en-us/sections/360003198300 for suggestions.

As mentioned earlier, this is the first time I am facing something like this!
Any ideas ?

[Juraj]

because you can upload without reset to bootloader if there is only the bootloader.
but normally to upload you have to reset the MCU to run the bootloader

[Watcher]

Yes I am aware of the need for a reset.
Reset is achieved via the TDR line of the external FTTDI module connected to the reset pin via an 100nF capacitor. See schematic below

By the way, I have also tried to manually reset the MPU at the start of the upload which no success.

[Watcher]

SOLVED!
I had to remove capacitor C19 in the schematic above.
Apparently it was interfering with the reset pulse from the FTTDI.
Now it all works fine.

Thanks for reading this post!"