Solved it with the help of the discord community.
The messages I was getting were coming from this part of the code:
https://git.kernel.org/pub/scm/network/tftp/tftp-hpa.git/tree/tftpd/tftpd.c#n1507
I was suggested that I should change the permission of the /config directory and the files, from in inside the container, to 777 and 666 respectively. This suggestion came from the comment at the top of that validate_access functions.
Indeed the global read permission vanished but then I started getting:
PXE-T00: Permission denied
PXE-E36: Error received from TFTP server
PXE-M0F: Exiting PXE ROM.
It seemes that chmod 775 -R /config did the job and finally it worked.
Ref: TFTP Permission denied - TFTP - openmediavault
I also noticed that any changes in files like e.g boot.cfg rendered permission errors during network booting.
I had to chmod 775 -R path-to/config on the host too in order to solve this.
I do not know if this is a Qnap specific issue or something in my configuration in general.
Special thanks to banadushi and antonym. Thanks guys.