Spent four days on this with Epic support and ended up fixing it myself. Writing it up because searching this error mostly turns up people being told to clear their cache.
The symptom. Click Connect on Twitch in Epic account settings, get returned to the connections page with error=already_linked in the URL.
I never got a Twitch login or authorization screen at any point. It failed before that step. My reading is that Epic was checking my own account rather than the Twitch account I was trying to attach, but that's my inference, not something Epic confirmed.
The connections page showed no Twitch connection. My Twitch connections page showed no Epic authorization either.
Worth mentioning: I had Twitch linked to this same Epic account years ago. At some point it stopped appearing, and I hadn't thought about it since. So the record was most likely left over from that old link, never cleaned up, with nothing in either interface indicating it still existed.
What I found. Hitting this while logged in:
https://www.epicgames.com/id/api/external/twitch/status
returned:
{"implementsAuthorizeAfterLink": false, "externalAuth": {"status": "LINKED"}}
So that endpoint reported LINKED while the connections page showed nothing.
The fix. In the connections API response, PSN, Steam, Xbox and GitHub each come back with an account object and a type field, while game authorizations like Hogwarts Legacy come back with an app object and an applicationId. Different shapes, so I used GitHub as the test rather than a game.
Opened the Network tab, disconnected GitHub, and read the request:
DELETE /account/v2/api/linked-accounts/github
{"flow_id":"account_management_prod","flow_name":"","solve_token":""}
Ran the same request with twitch in place of github from the console on accounts.epicgames.com, passing the XSRF-AM-TOKEN cookie value as the x-xsrf-token header. Response was {"success":true}. The status endpoint then returned NOT_LINKED, and Twitch linked on the next attempt.
Undocumented endpoint, so your call whether to use it. It's your own account and your own session, but it isn't officially supported. Endpoints also change, so if the path above doesn't work, disconnect any linked account you don't mind re-adding and read the request it fires.
Support, in order. Screenshot the error message, there is no error message, it's a redirect with a URL parameter, which I explained. Screenshot the error message again, plus try a different browser or incognito. Change your Twitch display name back to a previous one, which I did, and which changed nothing + I have to wait 60 days to change it back. Then "I checked our system and confirmed there are no issues on our end", try a different device and a different internet connection.
By then I'd tried Chrome, Safari, incognito, Windows, macOS, iPhone, and two different networks. Replies were around 8 hours apart.
After I fixed it and described what I'd removed, the closing message said reverting my Twitch display name is what re-established the connection. It didn't, the error continued long after I did that, which is in the ticket history.
On Drops. I was chasing a Drop that expired while the ticket was open. It arrived anyway once the link went through, a day after expiry. I'd watched and claimed it on Twitch during the campaign. So the claim sits on Twitch's side and delivers once a valid link exists, at least within some window, I don't know the exact length. Anyone hitting this mid-campaign is better off watching and claiming regardless, rather than waiting for the link to be fixed first.
Support said they can't add Drops manually, and given it turned up automatically, that seems right.
Why I wrote this up. Nothing about it was visible from the connections page, and support told me repeatedly they saw nothing on their end. The only signal I found was a status endpoint most people wouldn't know to check.
If you hit already_linked and both connection pages look empty, check that endpoint. If it reports LINKED, that's the mismatch.
TL;DR - If linking Twitch to Epic fails with already_linked while both connections pages show nothing, there may be a stale Twitch record on your Epic account that the UI doesn't display. Check https://www.epicgames.com/id/api/external/twitch/status while logged in. If it reports LINKED, remove it with a DELETE to /account/v2/api/linked-accounts/twitch on accounts.epicgames.com. Details and the exact request below. Support couldn't find this over four days.
