grimlord1

When I was reading the code it looked like the theme was having an issue.

Follow the steps here :https://www.wpbeginner.com/wp-tutorials/how-to-fix-pluggable-php-file-errors-in-wordpress/

see if this fixes it.

Try deactivating all of your plugins to explore whether any could be responsible. If none are, keep all plugins deactivated.

If you cannot get into the dashboard you can deactivate plugins via FTP. To deactivate all plugins, go to the /wp-content/plugins folder and rename plugins to plugins-old.

It looks like the Thumbnails are not generating, depending on the plugin is there an option to render thumbnails? And if you remove the plugin and create a post with an image, does it show?

I have had luck with https://www.wonderplugin.com/wordpress-video-player/ however there are thousands of plugins on the WordPress Plugin Repository. https://www.ads-software.com/plugins/ Search for Video.

To install the plugin you would go to the WP-Admin which should be

https://xn--aktivitetsjonglr-zxb.dk/wp-admin.php and log in with your email and password.

On the left, you should see PLUGINS
> Then continuing on the top left you will see ADD NEW. This is where you will install the new plugin. It’s drag and drop.

MyBB will continue using the same DB, it will just add another prefix.

Looks like the theme itself is having an issue, if you rename the Theme via FTP and then access does it resolve?

Curious, if you were to log in via FTP to the directory can you edit the files? Depending on how the folders are arranged that could be the issue.