During upgrade process I get following errors

[13-Aug-2017 11:04:40 UTC] PHP Fatal error: SQL ERROR [ postgres ]<br /><br />ERROR: sequence "phpbb3_styles_imageset_data_seq" does not exist []<br /><br />SQL<br /><br />DROP SEQUENCE phpbb3_styles_imageset_data_seq;

in /var/www/html/phpbb/db/driver/driver.php on line 999

for phpbb3_styles_imageset_data_seq, phpbb3_styles_imageset_seq, phpbb3_styles_template_data_seq, phpbb3_styles_template_seq, phpbb3_styles_theme_seq sequences.

It looks that these sequences may drop automatically after DROP TABLE <tablename>; because these errors happen only once per sequence; after restarting migration process everything goes fine.

To prevent these errors we can use DROP SEQUENCE IF EXISTS as a more robust solution.