Magento 2 "Unable to Unserialize Value. Error: Syntax Error": The Complete Fix
The Magento 2 "Unable to unserialize value. Error: Syntax error" almost always means an old PHP serialize() value is sitting where Magento now expects JSON, usually after an upgrade or a mismatched database restore. This guide fixes it fix-first for 2.4.4 to 2.4.9: flush the cache, read the stack trace to find the table, then convert or clear the one bad row safely.