I am also having the same issue. My user is trying to access his site, and he gets a database upgrade required message. Clicks on 'update' and this is the error he gets.
Catchable fatal error: Object of class WP_Error could not be converted to string in /data/15/2/74/79/2726405/user/2995896/htdocs/wordpress1/wp-admin/includes/upgrade.php on line 1352
Line 1349 - 1353 of updgrade.php says
"SELECT comment_ID FROM $wpdb->comments
WHERE comment_date_gmt > '2015-04-26'
AND CHAR_LENGTH( comment_content ) >= $content_length
AND ( comment_content LIKE '%<%' OR comment_content LIKE '%>%' )"
);
I can get to the backend of the install using FTP, fish out the code and put it into Dreamweaver to see it, but I don't know php and I don't want to alter it without some expert advice.
Help!