Another summary because nobody seems to have a clue but I keep trying.
The error occurs only when Fortnite is running in-session (e.g. for memory calculation with FN open).
Private version uploads succeed when Fortnite is not open.
The issue persists across two different PCs, with clean environments and cleared local Saved and Intermediate folders.
Revision Control has been toggled off/on without resolution.
I also tried reverting to old snapshots (even previously published ones), but the same error still occurs.