Error code 91 in Fortnite shows up when the game cannot complete joining another player's party, usually with a message that the party failed to respond or the join did not go through. It belongs to Epic's party service rather than your account or files: stale party state, privacy settings blocking joins, cross-platform friction, or Epic's services having a rough moment produce it most often.
Nothing about error 91 lives in the game files. It is generated by the handshake between two accounts' party states and Epic's service, so reinstalling changes nothing unless the local session was the stale half. The experiments that work, restart both sides, change who hosts, change how the invite happens, all operate on that handshake, which is where the fault actually sits.
Can my friend join me but I cannot join them. Why? One direction working proves accounts and connections are basically fine; the failing side usually has the stale party state or the stricter privacy setting. Restarting and toggling the party to public on the host side is the targeted fix.
Is error 91 a ban or shadow restriction? No. It is a party connection failure with no moderation meaning. Account actions from Epic arrive as explicit notices, not join errors.