Form is stuck on ‘processing’ with server and JS errors

I’ve noticed all of m ninja formes stopped working. After submitting, the forms are stuck on processing and I am seeing these messages in the console:

ailed to load resource: the server responded with a status of 500 () admin-ajax.php:1

ERRORS: 
front-end.js?ver=3.4.30:3 {readyState: 4, getResponseHeader: ?, getAllResponseHeaders: ?, setRequestHeader: ?, overrideMimeType: ?,?…}abort: ? (a)always: ? ()complete: ? ()done: ? ()error: ? ()fail: ? ()getAllResponseHeaders: ? ()getResponseHeader: ? (a)overrideMimeType: ? (a)pipe: ? ()progress: ? ()promise: ? (a)readyState: 4responseText: "<p>There has been a critical error on your website"setRequestHeader: ? (a,b)state: ? ()status: 500statusCode: ? (a)statusText: "error"success: ? ()then: ? ()__proto__: Object
front-end.js?ver=3.4.30:3 Parse Error

Uncaught TypeError: Cannot read property 'nonce' of undefined
    at i.submitErrors (front-end.js?ver=3.4.30:2)
    at p (backbone.min.js?ver=1.4.0:2)
    at f (backbone.min.js?ver=1.4.0:2)
    at l (backbone.min.js?ver=1.4.0:2)
    at u.Channel.n.trigger (backbone.min.js?ver=1.4.0:2)
    at Object.error (front-end.js?ver=3.4.30:3)
    at i (jquery.js?ver=1.12.4-wp:2)
    at Object.fireWith [as rejectWith] (jquery.js?ver=1.12.4-wp:2)
    at x (jquery.js?ver=1.12.4-wp:4)
    at XMLHttpRequest.c (jquery.js?ver=1.12.4-wp:4)

Everything is up to date, including the plugin. I have already tried disabling everything except the Ninja Forms plugin, but the problem still persists. I am using a SMTP settings in WP and the site is sending out e-mails correctly. I can see the form submissions in the WP admin, but no e-mails are being sent, and no confirmation messages display.

Is anyone else having this problem?