I get the same error generally on two different occasions, both of which are definitely SE's fault.
The first instance is, as with others, when I try to view a linkshell with large amounts of members and as the list is attempting to load I'm booted from the game, the 30002 error occurs when I attempt to log on.
The second instance is more vague and just appears to be a connection loss however it results in the same error when attempting to log on of 30002.
Now I know in FFXI that when you were booted from the server for whatever the reason was an error would come up if you attempted to log on too quickly, however the error was a little more detailed and actually TOLD you that your character was still logged on.
I did manage to get lucky and disconnect near my husband who was watching my character and the lag between my connection dropping off and me re-loading the game, and when my character actually disappeared from his screen was quite large. That said, if it is an error to due with that then why not just put that on the error tag?
Also, upon one of these occasions, I tried logging in right after with a second character which I was able to do with no problem at all. That also says to me that the error possibly has to do with the "character hangup" as I like to call it.
This is definitely not a connection issue, I have troubleshooted everything and my local ISP knows me by first time based on the number of techs that have come out to visit my house when I told them there was something wrong and it turned out they had a part going bad and fixed it and thus so did my internet.
So please, Development Team, Techs, whoever it may be, don't brush us off. I know the vast majority of complaints you here are probably crap and based on simple user errors however this one I don't think is, and neither does the large portion of the community it's effecting.