Hello, unfortunately we have the same problem on a client’s website.
The website esserstefan.de contains several contact forms, unfortunately only the one on the start page works. All forms that are not on the start page run into an Ajax error.
We have already deactivated all other plugins and deleted all caches. Unfortunately always without success. The website runs on WordPress 5.8.1 and the latest Quform version. A downgrade to an older plugin or WordPress version also brought no success.
We ask for help in troubleshooting.