Despite the scary-looking syntax error, it looks like it’s basically just an out-of-bounds map transfer. The map it attempts to load is a blank parent to a sub-map for a specific basement scene, which seems to set a flag that doesn’t get checked anywhere else and also transfers the player right back to the same blank map rather than back to the basement entrance.
For now it seems the best workaround is to just ignore the stairs, but since it’s an unrecoverable error that forces the player to reload the game, it should definitely be a priority for fixing or removing for the next patch.