iirc, its case issue. according to loftar, he always distinguish capital from lowercase, but phpbb doesnt seem to want to and give you this error.
it happens when you pick an existing name with a different case. (like, loftar and Loftar)
if you dont want this to happen, search from hearthlings list and pick one that does not exist in any different capitalizations.
memberlist.php?mode=searchuseri think he mentioned of fixing it sometime, but we never know when itll be fixed.