Sorry to hear of the problem you are having.
Please don’t share sensitive information such as usernames or passwords on the forum as this is public forum so the information shared can be misused by others. You can share it privately by adding them in the box having text “Click here to share private content. Only you and forum moderators will be able see it.”
For please change your shared credentials immediately for safety.
Your shared credentials are not working for me in your site Authentication Required Popup.
It seems you a using older version of theme on your site so please update it to the below latest version as described here https://support.shufflehound.com/updating-theme/
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This can be due to plugin conflict on your site so please try temporary deactivating all plugins except Unyson plugin and see whether everything works fine and then enable the plugins one by one to see which plugin is conflicting if any.
The Internal server error is server related issue so you can try resolving this issue as described in the following page or contact your web host to resolve it.