CASE 1:
Check core_store, core_website and core_store_group tables.
SELECT * FROM core_store WHERE code='admin';
SELECT * FROM core_store_group WHERE name='Default';
SELECT * FROM core_website WHERE code='admin';
There is a possibility that in these tables Admin is created as 1 for id. It should be 0 in all cases.
UPDATE `core_store` SET store_id = 0 WHERE code='admin';
UPDATE `core_store_group` SET group_id = 0 WHERE name='Default';
UPDATE `core_website` SET website_id = 0 WHERE code='admin';
CASE 2:
In another case, you might have 0 for id but there can be multiple rows inserted with the same value.
Delete such duplicate rows from core_store, core_website and core_store_group tables and the issue will be solved.
