getServerSideProps() surfaces a ServerResponse object through the res property of its context arg. This object is not intended to be accessed or changed after getServerSideProps() resolves.
This is because the framework tries to optimize when items like headers or status codes are flushed to the browser. If they are changed after getServerSideProps() completes, we can't guarantee that the changes will work.
For this reason, accessing the object after this time is disallowed.
You can fix this error by moving any access of the res object into getServerSideProps() itself or any functions that run before getServerSideProps() returns.
If you’re using a custom server and running into this problem due to session middleware like next-session or express-session, try installing the middleware in the server instead of getServerSideProps().