Haven't had it, but then, I haven't tried recently. Just to make sure, though:
Have you already got any characters on Tribble? If yes, does your Tribble account have sufficient empty character slots (for the faction of the character you're trying to transfer) to take another one? If no, you might want to try deleting one of them first, or if you have Tribble-zen, buy a few more slots, and attempt to copy the character over again.
If that doesn't apply to you, it's as the site said. Please try again later.
How many Tribble slots are we allowed to have? At one point I had 4 characters on Tribble, then tried transferring another and couldn't do it. I'm down to 2 and it still won't transfer another character.
Could it be I'm using items and ships Tribble doesn't have or support? (like Fleet Tribble or the Heavy Escort Carrier)
Yup can't copy any of my KDf characters...deleted existing copies from previous testing, but no more copying for me here too. I attempted this several times while the servers were up for more then a week now.