Looking at your site, I’m seeing an error that precedes the one for the modal. I’m thinking the modal issue might be a side effect of this first error that shows up in the console:
mc-validate.js:195 Uncaught TypeError: Cannot read properties of undefined (reading 'replace')
at Object.getAjaxSubmitUrl (mc-validate.js:195:14)
at mc-validate.js:351:11
at mc-validate.js:373:2
The mc-validate.js script is introduced in your theme here.
It might be worth trying to temporarily remove that script to see if it fixes the modal issue. If it does, you’ll likely need to do some digging to understand why the third-party script is failing.