Cannot decline war warning during autoexplore upon entry to enemy land (popup loop) (1) (2) (3)
Cannot accept war warning during manual move into emeny land (popup loop) (1) (2)
I also have regurlarly this problem. Waiting a turn or two will let you try and succeed, but i wasn't able to reproduce the problem when I want. Sometimes you just have to wait one turn, sometimes a lot of turns.