This is most probably due to a JS conflict on your website, please try the following:
- Ensure everything is up to date according to our version compatibility list here. Please follow the best practices when updating your theme and plugins. Click here for more information.
- If you're using a CDN, please clear the CDN's cache and disable optimization services.
- Test for a plugin conflict. You can do this by deactivating all third-party plugins, and see if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
- Increase the PHP Memory Limit of your server. Click here for more detailed information and how to increase the PHP memory limit.
If none of these helped, then please share your WordPress Dashboard login details in a "Secure Note" so we can investigate this issue.