It's going to be difficult to find the problem this way. The non-protected nature of the Amiga's memory means it could be anything going bad and causing that crash, the debug information won't tell you much (just what program was unlucky enough to have itself overwritten by the rogue task).
Taking a different tack, you have noticed it happens with YAM when downloading mail. So in all probability YAM or one of its components is the culprit! Try and make it reproducible and submit it as a bug report. (I also notice you are not running the latest YAM version - perhaps it is fixed already!)
