I have the same issue that this person did, though I'm updated to 4.6.10 The issue is that when submitting payments, I get the 9003 error code, which says "MD5 Verification failed." I've reset the MD5 code from both sides, and am confident that they match. It's possible that my issue could be solved with the answer that I linked, but I don't know enough to apply it. It's also possible that Authorize.net tech support could help me, but they won't do anything without the log and I'm unsure where it lives. So...
1) Do you know how to fix this error?
Or failing that,
2) Where are CiviCRM's payment processor logs?