Recoverable fatal error: Argument 3 passed to l() must be an array, null given

This is not a bug. Argument 3 of l() is optional, meaning it may very well be NULL.

Please install the Devel module and enable it's Backtrace error handler, so that we can see the call stack leading up to the error. Post a screenshot of that.

Hmm, let's see...

The "Argument 3 passed to l() must be an array" message is really strange. Typically, this is generated by PHP, but if the signature of l() is l($text, $path, array $options = array()) as api.drupal.org claims, then the argument 3 is optional. However, line 2583 in menu.inc is...

    foreach ($active_trail as $parent) {
      $breadcrumb[] = l($parent['title'], $parent['href'], $parent['localized_options']);
    }

... and $parent['localized_options'] us undefined, so NULL is passed (rather than nothing), which leads to the error. We can try to do some more sleuthing.

Temporarily put a dpm($active_trail); above the foreach, open the array by clicking on it and look for an element that does not have any 'localized_options' key. I have no idea where those keys are coming from, but knowing which element in the array is missing may give us some more information.

When I do this on my test installation, I'm seeing an array with five elements, and each one is another array (click to expand), and each of those has a 'localized_options' key. What do you see?

(Wouldn't null be fine for a truly optional argument?).

No. NULL is not nothing. If nothing is passed, then PHP adds the default value, which is array() in this case (an empty array). An empty array is not the same thing as NULL.

You're on the right track, and this may turn out to be a Mail Edit bug after all.

If your problem user does not have access to the administrative pages, he should not have access to admin/config/system/mail-edit either, because it's an administrative page.

Mail Editor should not give access to admin/config/system/mail-edit to someone who does not have the "Use the administration pages and help" permission. This is a bug in Mail Editor.

ME's "Administer mail templates" even includes the magic word "administer" — this should be enough of a hint that "Use the administration pages and help" is also needed, because it applies to all "Administer something" permissions. Depending on your theme, if you're using an admin theme, "View the administration theme" would probably be useful or even necessary as well. I've added this information to the README.txt file.

The "Use the administration pages and help" has no security implications. There's no reason to not give it to your problem user.

Thank you for your help and perseverance in tracking this down. ME's failure to check the "Use the administration pages and help" permission allowed you to get into a situation where core was not behaving properly. Since you can't normally get there, it's a non-issue for core, so no further action is needed. BTW, I was eventually able to reproduce this error, even without having i18n installed.

I've committed the fix to the D7 and D6 -dev versions.

Thanks!

When it is needed to grant access to ONLY template editing, it is not good to grant "Use the administration pages and help" permission. According to your source code this permission is used to build breadcrumbs, which could be absent on this page.
We suggest not to add a forbidden item to breadcrumbs instead of making the whole page not available. This patch implements this. Please review it.

#11:

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