I have this same error. However only when I add www in front of my domain. https://sellemental.com produces no error https://www.Sellemental.com does.
Warning: Class __PHP_Incomplete_Class has no unserializer in /home/content/p3pnexwpnas13_data02/77/3033977/html/wp-content/object-cache.php on line 520
Warning: Cannot modify header information – headers already sent by (output started at /home/content/p3pnexwpnas13_data02/77/3033977/html/wp-content/object-cache.php:520) in /home/content/p3pnexwpnas13_data02/77/3033977/html/wp-includes/pluggable.php on line 1171
GoDaddy is claiming it’s not a GD problem but all posts seem to point only them.
???