Launching UEFN project gives matchmaking error - "player banned from sub game"

Summary

I have been working on a UEFN project and launching sessions from the editor all day without a problem. Suddenly I have started getting a matchmaking error when I try to launch session:
errors.com.epicgames.fortnite.player_banned_from_sub_game
BuildIdOverride is: 39162266

I tried loading Fortnite and got a warning saying “due to recent actions on your account, you have been warned for: using third-party tools to unlock cosmetic items”.
I have done nothing of the sort, I am just trying to work on a UEFN project.

After clicking past the warning I was able to play a random Fortnite island. I went back to the editor, tried to launch my project again and got the same error message. Now when I go back to Fortnite it says I have been banned for 24 hours.

Can someone please help me resolve this, it is stopping me from working, thanks.

Please select what you are reporting on:

Unreal Editor for Fortnite

What Type of Bug are you experiencing?

Matchmaking

Steps to Reproduce

Load a project into UEFN
Attempt to launch it by pressing Launch Session

Expected Result

The session launches OK

Observed Result

I get a matchmaking error

Platform(s)

Windows PC

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