Yes, those session.php line number offsets match expected offsets for phpBB 3.2.0. And indeed, phpBB 3.2.0 is not compatible with PHP 7.2, and required PHP 7.2 fixes which were delivered in phpBB 3.2.2 and later. Updating to phpBB 3.2.8 would be a great way to address the issue, rather than backing off the PHP version to 7.1.x.
If you are trying to use the automatic update process, it might be dying for PHP 7.2 compatibility issues just like the live board itself is reporting issues. If it's not already what you were doing, use the full package download instead of the automatic update, and use the Updating 3.2.x
process to perform the update, to help ensure you're running PHP 7.2-compatible code during the update.
Or, if the full package update process is already what you were attempting when seeing the failure, find your PHP error log to see what PHP error was occurring when hitting the "blank page" result. Your PHP error log might simply be an error_log file visible to you through FTP, or your hosting provider may have provided access to the PHP error log in some other file, or through your hosting control panel.
The PHP error log would show the specific error and potentially a call stack for how it was reached, if there is some kind of extension or disabled PHP feature that is causing the execution to not be able to proceed normally.