There have been a number of reports of tasks crashing immediately after upgrading to BOINC 6.12.26 (here, here and here).
Clicking on the '+' on the stderr line of the task page shows that affected tasks fail with an error message similar to the following:
execl(/Library/Application Support/BOINC Data/projects/climateprediction.net/hadcm3n_um_6.07_i686-apple-darwin, 130475) failed!
Mac runs applications in a sandbox, with the BOINC core client running project applications under a limited permissions account. All CPDN applications consist of 2 programs; BOINC runs the controller process with starts the worker process (e.g. hadcm3n_um_6.07_i686-apple-darwin). It appears that a permissions change in 6.12.26 is preventing the worker process launch when the application programs are already on your system.
Resetting CPDN (or detaching and reattaching) forces the application programs to be redownloaded and should fix the problem.
"The ultimate test of a moral society is the kind of world that it leaves to its children." - Dietrich Bonhoeffer