The stderr messages for your failed tasks all contain errors similar to the following:
execl(/Library/Application Support/BOINC Data/projects/climateprediction.net/hadcm3n_um_6.07_i686-apple-darwin, 137095) failed!
Controller:: CPDN process is not running, exiting, bRetVal = 1, checkPID=0, selfPID=10400, iMonCtr=1
Model crash detected, will try to restart...
That computer's task list has a HadAM3P_SAF task crashing with the same error on 26th May and a HadAM3P_EU task completing successfully on 22nd May. The other notable difference between them is that the EU task reported BOINC core client version 6.10.58 and the SAF one reported 6.12.26, so the error could indicate that the upgrade caused problems with the permissions.
Interestingly, the SAF task returned 5 trickles after the error was reported, suggesting that you managed to get it to run from a backup. How did you manage that?
"The ultimate test of a moral society is the kind of world that it leaves to its children." - Dietrich Bonhoeffer