What is the closest noun for "that which confirms"?
Solution 1:
Is the answer not confirmation. It is certainly "that which confirms".
It also works with your example sentence: The confirmation was broken, and clients were unable to establish the requests were submitted.
Solution 2:
Word Hippo has
confirmator
One who, or that which, confirms; a confirmer.
and
confirmer
One who confirms something
along with lots of others, like
confirmee
A person who is confirmed via religious rite.
and
confirmand
A candidate for confirmation or affirmation of baptism.