My games had a bug that Solarus failed to detect before 1.6 (bad parameter passed to sol.timer.start). Solarus 1.6 detects it and shows an error message.
To fix it, you need to do the same change as here: https://gitlab.com/solarus-games/zelda-xd2-mercuris-chess/commit/f812e2ae52a8e78968d4a61bc61ecb48b8d4d347
To fix it, you need to do the same change as here: https://gitlab.com/solarus-games/zelda-xd2-mercuris-chess/commit/f812e2ae52a8e78968d4a61bc61ecb48b8d4d347