The obvious answer is to treat carob like chocolate ... use a double boiler ... over simmering (not boiling) water so it melts gently.
If it get's any water, or anything cold, in it - it could get grainy or seize.
A tablespoon or two of heavy/whipping cream (cream - not 2% low or no fat milk - real cream) or butter might also help with the grainyness - added to the pot before you start heating it.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain