I'm trying to start a new game from FEN but I must be missing something. I copied the FEN-string from a position in another game, so it should be ok?
No luck starting a new game from FEN


This may help: https://support.chess.com/customer/en/portal/articles/1444914-what-is-pgn-fen-
Basically, you have to include the side to move, at the very least, and potentially castling rights and if en passant is possible. You shouldn't need the move number, I don't think, but if you include it be aware that if you start after move 4 and the game is rated, you can't abort without losing rating (unless they have fixed it).