Access log file from failed upgrade from PostgreSQL 10.7 to 11.2.

Trying to upgrade from PostgreSQL 10.7 to 11.2 with default configuration.

The upgrade runs for some times and then reverts back to 10.7 (luckily). The log file contains:

*failure*
There were problems executing ""/rdsdbbin/postgres/bin/pg_restore" --host /rdsdbdata/log/upgrade --port 8193 --username rdsadmin --create --exit-on-error --verbose --dbname template1 "pg_upgrade_dump_16404.custom" >> "pg_upgrade_dump_16404.log" 2>&1"
Consult the last few lines of "pg_upgrade_dump_16404.log" for
the probable cause of the failure.
"/rdsdbbin/postgres/bin/pg_ctl" -w -D "/rdsdbdata/db" -o "--config_file=/rdsdbdata/config_new/postgresql.conf" -m fast stop >> "pg_upgrade_server.log" 2>&1

*failure*
There were problems executing ""/rdsdbbin/postgres/bin/pg_restore" --host /rdsdbdata/log/upgrade --port 8193 --username rdsadmin --create --exit-on-error --verbose --dbname template1 "pg_upgrade_dump_13240846.custom" >> "pg_upgrade_dump_13240846.log" 2>&1"
Consult the last few lines of "pg_upgrade_dump_13240846.log" for
the probable cause of the failure.
"/rdsdbbin/postgres/bin/pg_ctl" -w -D "/rdsdbdata/db" -o "--config_file=/rdsdbdata/config_new/postgresql.conf" -m fast stop >> "pg_upgrade_server.log" 2>&1
child process exited abnormally: status 256
"/rdsdbbin/postgres/bin/pg_ctl" -w -D "/rdsdbdata/db" -o "--config_file=/rdsdbdata/config_new/postgresql.conf" -m fast stop >> "pg_upgrade_server.log" 2>&1
----------------------- END OF LOG ----------------------

However, the file pg_upgrade_dump_....log can not be found in the user interface.

Does anyone know how to find the root cause of this failure to restore the backup and finish the upgrade?