Can't join my unpublished island via Launch Session — Fortnite matchmaking error and crash

Summary

Hello! I’m facing a critical issue with UEFN.

My island hasn’t been published yet, so I’m relying on Launch Session to test and edit.
However, I’m unable to access my island at all. Below are the main errors I’m facing:

Please select what you are reporting on:

Unreal Editor for Fortnite

What Type of Bug are you experiencing?

Other

Steps to Reproduce

1-Click “Launch Session” in UEFN to start an editing session for an island that has not yet been published.

2-Wait for Fortnite to open and attempt to pair with the session.

Expected Result

That Fortnite loads the island normally.

That the matchmaking with the session works and I can enter the island to edit and test.

Observed Result

In some attempts, the error appears:
“Client failed to register with the server. Please restart Fortnite.”

In others, it appears:
“Pairing error: Session ID not found.”

When the connection seems to work, Fortnite freezes on a loading screen with the message:
“Joining your editing session.”

And in some cases, Fortnite simply crashes, displaying:
“Fortnite has crashed.”

Platform(s)

PC

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Additional Notes

The island is not published, so I rely on Launch Session to test.

I have already restarted Fortnite and UEFN, checked the connection and the problem persists.

This is completely blocking my creation process.