I create trade guilds in my alliance and have never experienced this kind of problem before, but it seems that one of the trade partners can't accept/offer the 5th trade to complete the guild, even though he has a Harbor. It just keeps telling him that he's over the limit.

This is his nation: http://www.cybernations.net/nation_drill_d...ation_ID=165328

and this is the last nation he needs a trade agreement with: http://www.cybernations.net/nation_drill_d...ation_ID=352056

Does he have any open trade offers from others? He needs to cancel those first.

He has 4 active trades, 0 pending. At first he couldn't even receive and accept 4 trades, so he tried to send the trade offers to the other people instead. It only partially worked, and he's still missing the 5th trade.

He needs to delete any trade offers that say "Cancelled" on his Trade Agreements screen.

Normally cancelled trades expire automatically shortly after they are cancelled (if you don't manually delete them), and shouldn't prevent anyone from sending/accepting trade offers.

Anyway, he now says that he was able to send the trade offer to the 5th person, but although the recipient accepted the trade offer it's not in the trade screen. It's just as the trade agreement vanished.

Could a mod please solve this?

No...only admin can.

He had more than 5 trade offers. He just needed to delete the trade offer that he didn't want and accept the other one.