Although as far as I know all plunder quests now have alternatives as well, there is something more to it in this case. Please remember that this is Early Middle Ages content - and it does not take into account that you have nothing left to research: you've only just begun the game! ... If there is no abort or skip option, and they cannot set up an agreement with someone else who's in the same boat, they may well have to wait until the next era will be released with new research. When that will be - I have not a single clue, I'm afraid.