ehh, seems there's typo on that error message. should be 'mob_readdb_itemratio'
well, and then it's because mob.c reads the files with this sequence.
read db/re/mob_db.txt read db/mob_item_ratio.txt read db/import/mob_db.txt read db/import/mob_item_ratio.txt
when u put item in db/mob_item_ratio.txt for custom monsters that written in db/import/mob_db.txt, of course map-server doesn't recognize them.
Try put ur custom mobs & custom mob_item_ratio on import files.