customer_group table in Magento2 database.
Check NOT LOGGED IN value in
customer_group_code field. If it's not available in that field, that means when the customer Sign Out then Magento2 can't get the customer group for
NOT LOGGED IN group.
NOT LOGGED IN group in the database, Execute this in SQL,
INSERT INTO `customer_group` (`customer_group_id`, `customer_group_code`, `tax_class_id`) VALUES ('0', 'NOT LOGGED IN', '3');
And if 'NOT LOGGED IN' value available in that field then set its
UPDATE `customer_group` SET customer_group_id = 0 WHERE customer_group_code='NOT LOGGED IN';
This work for me properly.