Multisite Wordpress can't create new posts, can not add to cart.

I need an experienced person who knows how multisites work in wordpress
and who already knows this type of problem because it is very urgent for me.
So if you already know this type of problem it will be easier I am at 1and1 and all plugins and wordpress (latest version)

I have run into a problem where
I can no longer create new posts on my Wordpress site.
I can't add a new user
I can not add to cart
...


Please save before modifying if you are not sure because it is a multisite ecommerce data is sensitive

I think this started after a Wordpress update a while ago. I also have problems uploading images and I see ghost comments (an icon about new comments but no comments actually showing up in the list).

When clicking on "Posts > Add new" I get a white page with this warning:

"Warning: Creating default object from empty value in /public_html/wp-admin/includes/post.php on line 716"

This is line 716:

$post->post_content = (string) apply_filters( 'default_content', $post_content, $post );
After searching online I think these issues are related to my database. I'm new to dealing with databases, so I'm hoping to maybe be able to get some help here.

Things I've tried:

Using a standard theme
Disabling all plugins
Updating Wordpress
Downgrading to Wordpress 4.0
Repairing the database tables
Running health check I found out that the SQL is old (5.5.52-MariaDB-cll-lve). Could this be relevant?
Checked the AUTO_INCREMENT box in phpMyAdmin for "wp_posts > structure > ID" and "wp_postmeta > structure > idmeta_id"
The auto_increment helped remove the error, but instead it caused some new problems. I activated it and could click "Posts > Add new". However, Wordpress created multiple copies of the same post, around fifteen posts instead of just one. I trashed them and tried to create a new post, but it still remembered the trashed post and tried to recreate a new post from the post in the trash.

Links I've checked and tried:

https://wordpress.org/support/topic/creating-default-object-from-empty-value-of-line-639-wp-adminincludespostphp/
https://wordpress.org/support/topic/shoud-wp_posts-id-and-wp_post_meta-meta_id-be-auto_increment/
I feel like this could be relevant as well, but I'm not sure how to make use of it:

https://wpindexfixer.tools.managedwphosting.nl/
Super thankful for any help!


I also have a fatal error

I have for the moment commented on this line

/wp-content/plugins/woocommerce/packages/action-scheduler/classes/migration/ActionScheduler_DBStoreMigrator.php sur la ligne 44

/*throw new \RuntimeException( sprintf( __( 'Error saving action: %s', 'woocommerce' ), $e->getMessage() ), 0 );
*/


Erreur fatale: Uncaught RuntimeException: Erreur lors de l'enregistrement de l'action: Erreur lors de l'enregistrement de l'action: Commande INSERT refusée à l'utilisateur 'o316832'@'infong-eu180.clienthosting.eu' pour la table 'wp_30_actionscheduler_actions' dans /homepages/22/d820398088/htdocs/lynk/wp-content/plugins/woocommerce/packages/action-scheduler/classes/migration/ActionScheduler_DBStoreMigrator.php:44 Trace de pile: # 0 / homepages / 22 / d820398088 / htdocs / lyn wp-content / plugins / woocommerce / packages / action-scheduler / classes / data-stores / ActionScheduler_HybridStore.php (242): ActionScheduler_DBStoreMigrator-> save_action (Object (ActionScheduler_Action), NULL) # 1 / homepages / 22 / d820398088 / htdocs lynk / wp-content / plugins / woocommerce / packages / action-scheduler / classes / ActionScheduler_ActionFactory.php (177):ActionScheduler_HybridStore-> save_action (Object (ActionScheduler_Action)) # 2 /homepages/22/d820398088/htdocs/lynk/wp-content/plugins/woocommerce/packages/action-scheduler/classes/ActionScheduler_ActionFactory.php(84)/homepages/22/d820398088/htdocs/lynk/wp-content/plugins/woocommerce/packages/action-scheduler/classes/migration/ActionScheduler_DBStoreMigrator.php sur la ligne 44