Hello there,
Since we have no other reports of this issue form our other users and since we cannot replicate this issue in our local installation of WordPress, this should be something unique to your installation caused by a theme or plugin conflict.
We need to find out what is causing the conflict in order to be able to sort out this issue.
To search for conflicts, please follow these steps.
1. First please deactivate all plugins except Optin Cat and see if the issue persists.
2. If the issue is resolved, reactivate your plugins one by one to see if the issue returns.
3. If no conflict is found among your plugins, try reverting to a default WP theme like ‘Twenty Sixteen’, or ‘Twenty Seventeen’, and see if the issue is resolved.
Head here for detailed instructions:
https://fatcatapps.com/knowledge-base/search-for-conflicts/
If you find a conflict with a plugin or theme, please let us know the name of that software and also try to contact the author of the conflicting software to see if they can fix the issue.
Thanks.