Drupal 7 - Array to string conversion in DatabaseStatementBase
Webchick's Quick-and-dirty debugging is so useful in this type of situation that I've sometimes had a projects with it as a ready to apply patch.

Find drupal_set_message()

function drupal_set_message($message = NULL, $type = 'status') {


in bootstrap.inc, and add

// DEBUG: Go track down that stinkin' error...
if ($type == 'error') {
  $message .= '&lt;pre&gt;'. print_r(debug_backtrace(), 1) .'&lt;/pre&gt;';
}
// END DEBUG


at the very top. This will give you a backtrace on the error, helping you to trace down it's source. It could be good to know that there are instances where this runs PHP out of memory, if you have a lot of errors, so you may need to add extra conditionals in there at times.