It seems like server timeouts interrupt upgrade process.
Please contact your server administrator and ask to increase the max_execution_time and memory_limit server PHP directives. Also if mysqldump is available at your server, you can set the value of the backup_db_mysqldump tweak in the config.local.php file to true:
'backup_db_mysqldump' => true,
This will significantly speed up the backup process, but may not be supported by the server you are using.