Why does that happen?
When you see such an output, then it normally means, that at some point - in some codepiece - there's a function not only (silently) returning something, but
echo/print-ing something. When
lambda_* is actually the output printed to the screen, then I'd say, it's a function defined inside another function that is returning something by accident.
Debugging and Backtracing
As it happens on the login screen, I'd suggest to take a look at the
wp-login.php file, search for all
apply_filters appearances. Write them down and then do a cross file search in your plugins and/or theme and check if you got a callback somewhere attached to one of those hooks or filters. Now go and check all of those and take a look if one of those functions has either a lambda
function inside or calls to other (non-core) functions, that have such a call.
When you're done and have found all those lambda functions, then inspect which one of them
prints something. If you're still left with more than one, then there's a high chance that you can determine the error prone function using your brain and read its name, or just dig into it and kill the
echo/print one by one (and reload the login page on each try) until you've found the function causing it.
What to do now?
That's pretty easy: Contact the author with a detailed problem & error description and with steps that show how you fixed the bug.
The (happy) End.