Likely a low RAM issue. I managed to reproduce this using a Whonix VM with default 2 GB RAM.
While trying to debug kswapd0 process had high CPU and system responded in time lapse.
Increasing VM RAM to 4 GB RAM and I am past that build bug.
root cause - maybe: cowbuilder or something using aptitude. And then aptitude, dpkg or postinst showing obscure error messages and no out of RAM error.