It seems if you have a STAT active relation which is MANY_MANY then Yii just dies and outputs ‘?’ - four question marks. No log is generated.
This has been resolved in another thread (http://www.yiiframework.com/forum/index.php?/topic/7637-cactivefinder-die/) , but the bug is still in the current version of 1.1.1 (r1907) that I just downloaded.
I just thought it was worth highlighting this in case anyone else is stumped by why their app is suddenly hard-failing.