I have the same outcome (ERR_NAME_NOT_RESOLVED when trying to open Fab window in-editor) with slightly different log entries.
LogFab: Display: Logging in using persist
LogWebBrowser: Loaded CEF3 version 128.4.13.3057 from
LogCEFBrowser: CEF GPU acceleration enabled
LogEOSSDK: LogEOSAuth: UpdateUserAuthToken: User ClientId: <deleted_by_me> AccountId: <deleted_by_me> Access[Expires: 2025.06.14-20.47.04 Remaining: 7200.36] Refresh[Expires: 2025-12-11T18:30:35.037Z Remaining: 15551011.04] State: Valid
LogEOSSDK: LogEOSAuth: UserAuthGenerated: Received FUserAuthToken
LogEOSSDK: LogEOSAuth: UserAuthGenerated: Login complete: EOS_Success
LogFab: Display: User logged in
LogFab: Display: User client id: [deleted_by_me]
LogEOSSDK: Warning: LogEOS: Error response received from backend. ServiceName=[Friend], OperationName=[GetBlockList], Url=[], HttpStatus=[403], ErrorCode=[errors.com.epicgames.common.insufficient_scopes], NumericErrorCode=[1056], ErrorMessage=[Insufficient access scopes. Expected: [friends_list]], CorrId=<deleted_by_me>
UE version: 5.6, downloaded from https://www.unrealengine.com/en-US/linux
Fab plugin was downloaded from the same location.
Linux: CachyOS current.
Thank you.