Hmm.... I had this bug just now, and it turns out (in my case) that the problem was some outdated code. rAthena trunk should work correctly, so perhaps this is caused by some custom code bug.
The specific issue was that I was using an older char-server codebase, and my char-server was sending back after authentication (0x2727 in parse_fromlogin) mmo_char_send006b() instead of mmo_char_send(). What this means is, if you just send 006b, you are only sending packet 0x6b (character list), whereas the server should send (for 2013 clients) packets 0x82d, 0x9a0, and 0x20d. In particular, I think the problem lies in packet 0x82d, because this is the one that sends in how many characters you are allowed to have (sd->char_slot, which defaults to MAX_CHARS if character_slots is zero in the account database).
In any case, rA-trunk should be correct, so I suggest you look closer at what packets are being sent/received so you can figure out where the actual bug is.
