I visited another guild member's island to buff some mines.
The first weird thing was that I saw his building queue (he was building a well; he confirmed when I asked).
Then he sent me some buffs I didn't have and I - still on his island - got a message "You do not have the resources to initiate this trade." He had no problems with the trade and they arrived in my mail all right.
Then I went home, accepted the trades and went back to continue buffing and the game froze. Chat was still alive, though. I logged out, which also worked, and afterwards everything seems all right.