Website Broken - Needs Fixing

My website has automatically been updated to wordpress version 5.6.1 by Ionos hosting.

As a result, the comet theme plugin (https://themeforest.net/item/comet-creative-multipurpose-html-template/13857797) is not working and the site is down.

I have had to deactivate the comet plugin because when it it is activated, the header comes up with code, and I can't login to the dashboard through /wp-admin.

Instead I just get the following warnings:

Warning: Creating default object from empty value in /homepages/34/d843778203/htdocs/app857904902/wp-content/plugins/comet_addons/admin/ReduxCore/inc/class.redux_filesystem.php on line 29

Warning: Cannot modify header information - headers already sent by (output started at /homepages/34/d843778203/htdocs/app857904902/wp-content/plugins/comet_addons/admin/ReduxCore/inc/class.redux_filesystem.php:29) in /homepages/34/d843778203/htdocs/app857904902/wp-includes/pluggable.php on line 1296

Warning: Cannot modify header information - headers already sent by (output started at /homepages/34/d843778203/htdocs/app857904902/wp-content/plugins/comet_addons/admin/ReduxCore/inc/class.redux_filesystem.php:29) in /homepages/34/d843778203/htdocs/app857904902/wp-includes/pluggable.php on line 1299

When I deactivate the comet plugin, the warnings go and I can login but the website just appears with code on it, like here: https://so-creative.co.uk/

Logins:

I've been told by ionos hosting it's because the comet plugin hasn't been updated to be compatible with the latest wordpress update.

Can anyone fix it / know how to fix it? Needs to be done ASAP