Well since I own a Nokia 6230 (not the i version unfortunately), I recommend you check its firmware version by typing *#0000# (no need to press the green button)
Somewhere I read that newer firmware fixes some decoder issues...
BTW, FWIW my firmware is v05.24 dated 25-11-04
(Truth be told, I now use my 6230 for Radio, and I play exclusively Ogg Vorbis files with my iPaq 2210 using GSPlayer)
Edit: See my AAC settings on my signature
