Can you update your Ninja Forms plugins to their latest versions? Can you then try temporarily deactivating all your plugins that are not Ninja Forms add-ons and switching to a default WordPress theme like Twenty Twenty.
If you do not have a development environment for your site (this is best practice and highly recommended), you can use the Health Check plugin (https://wordpress.org/plugins/health-check/) to temporarily disable plugins and your theme for a single user session, instead of all users using your site.
If this solves your issue, then the problem was caused by an issue in one of those plugins or your theme. Reactivate them one at a time until the issue returns to determine which plugin is causing the conflict.
If you are still having issues, can you contact us through or official support channel (ninjaforms.com/contact) so we can help you out with this issue.
