My take on this….
1) the “error message but change stored OK” thing seems to be a quirk of build 1586. Myself and several others have also seen it.
2) on a slug there is no log file by default as far as I know – but you can specify its location via the config file / config web page – make sure the path/filename points to somewhere on the hard disk
3) can’t help – sorry – I don’t use playlists