I also experienced this bug. I was running Ubuntu 18.04, and I upgraded to Ubuntu 20.20 yesterday by running `update-manager -d` on the command-line, which launched the GUI. During the upgrade itself, I was shown a dialog displaying an error relating to pam.

I've attached apt-term.log (for me) to this comment. Here is the relevant error, I think, I don't know how to find the core dump:

Preparing to unpack .../libpam0g_1.3.1-5ubuntu4_amd64.deb ...
Unpacking libpam0g:amd64 (1.3.1-5ubuntu4) over (1.1.8-3.6ubuntu2.18.04.1) ...
Setting up libpam0g:amd64 (1.3.1-5ubuntu4) ...
Fontconfig warning: "/etc/fonts/conf.avail/53-monospace-lcd-filter.conf", line 10: Having multiple values in <test> isn't supported and may not work as expected
GLib-GObject-WARNING **: specified class size for type 'PangoCairoFcFont' is smaller than the parent type's 'PangoFcFont' class size at /usr/lib/x86_64-linux-gnu/perl5/5.30/Glib/Object/Introspection.pm line 67.
GLib-GObject-CRITICAL **: g_type_add_interface_static: assertion 'G_TYPE_IS_INSTANTIATABLE (instance_type)' failed at /usr/lib/x86_64-linux-gnu/perl5/5.30/Glib/Object/Introspection.pm line 67.
GLib-CRITICAL **: g_once_init_leave: assertion 'result != 0' failed at /usr/lib/x86_64-linux-gnu/perl5/5.30/Glib/Object/Introspection.pm line 67.
GLib-GObject-CRITICAL **: g_object_new_valist: assertion 'G_TYPE_IS_OBJECT (object_type)' failed at /usr/lib/x86_64-linux-gnu/perl5/5.30/Glib/Object/Introspection.pm line 67.
dpkg: error processing package libpam0g:amd64 (--configure):
 installed libpam0g:amd64 package post-installation script subprocess was killed by signal (Segmentation fault), core dumped
Errors were encountered while processing:
 libpam0g:amd64
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(Reading database ... 276143 files and directories currently installed.)
Preparing to unpack .../libpam-modules-bin_1.3.1-5ubuntu4_amd64.deb ...
Unpacking libpam-modules-bin (1.3.1-5ubuntu4) over (1.1.8-3.6ubuntu2.18.04.1) ...
dpkg: dependency problems prevent configuration of libpam-modules-bin:
 libpam-modules-bin depends on libpam0g (>= 0.99.7.1); however:
  Package libpam0g:amd64 is not configured yet.

dpkg: error processing package libpam-modules-bin (--configure):
 dependency problems - leaving unconfigured
Errors were encountered while processing:
 libpam-modules-bin
dpkg: regarding .../libpam-modules_1.3.1-5ubuntu4_amd64.deb containing libpam-modules:amd64, pre-dependency problem:
 libpam-modules:amd64 pre-depends on libpam0g (>= 1.3.1)
  libpam0g:amd64 latest configured version is 1.1.8-3.6ubuntu2.18.04.1.

dpkg: error processing archive /var/cache/apt/archives/libpam-modules_1.3.1-5ubuntu4_amd64.deb (--unpack):
 pre-dependency problem - not installing libpam-modules:amd64
Errors were encountered while processing:
 /var/cache/apt/archives/libpam-modules_1.3.1-5ubuntu4_amd64.deb
dpkg: error processing package libpam-modules:amd64 (--configure):
 package libpam-modules:amd64 is already installed and configured
Errors were encountered while processing:
 libpam-modules:amd64