On this line:

https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/phpbb/db/migration/data/v310/style_update_p1.php#L164

it says:

			$sql = 'SELECT style_id

				FROM ' . STYLES_TABLE . "

				WHERE style_name = 'prosilver'";

			$result = $this->sql_query($sql);

			$default_style = $this->db->sql_fetchfield($result);

			$this->db->sql_freeresult($result);

I got the following error:

Geïnstalleerde data: \phpbb\db\migration\data\v310\extensions; Tijd: 0.70 secondes
Geïnstalleerde schema: \phpbb\db\migration\data\v310\style_update_p1; Tijd: 0.14 secondes
PHP Warning: Illegal offset type in isset or empty in /media/marco/websitesHD/websites/unixerius/git/phpBB31/phpbb/db/driver/driver.php on line 355
Geïnstalleerde data: \phpbb\db\migration\data\v310\style_update_p1; Tijd: 0.12 secondes
Geïnstalleerde schema: \phpbb\db\migration\data\v310\style_update_p2; Tijd: 0.26 secondes

This seems to trace back to the sql_fetchfield() call in line 163 mentioned above. I guess it's not $result that should be passed, but 0 or 'style_id' or sth like that?