Cannot post entries and comments

I cannot write post entries or comments anymore. When I click “new post” I get a white page (error 500). When I want to comment (no matter if I’m logged in or not), I get an error message “comment couldn’t be saved. please try again later.” I also can’t comment in the comment section of the backend. What I find remarkable is that I _can_ write and save via the Dashboard widget (quick entry). When I have the troubleshoot mode ON (via Health Check) and all plugins deactivated I can view a “new post” page, but I cannot save anything that I have written there and buttons to “publish” are missing.

What I did beforehand: Those last few days I did quite a lot in order to make my page gdpr-compatible. Until now I didn’t want to post anything on my page, so I found out about the problem rather late.
1. I got a SSL certificate. Therefore I also exported my database to find and replace “http” with “https”. Then I imported the database again and got the following error:
“SQL-Befehl:
ALTER TABLE x_options
ADD PRIMARY KEY (option_id),
ADD UNIQUE KEY option_name (option_name)
MySQL meldet:
#1062 – Duplicate entry ‘0’ for key ‘PRIMARY'”
Afterwards some part of my child theme was broken. I couldn’t use the Customizer to change font color and logo. I found a workaround.
2. Due to the SSL certificate I also made a change in my .htaccess file and added:
“RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]”
3. I also uploaded lots of plugins. But I already tried the troubleshoot mode and deactived all and the only thing that changed was what I described above. The plugin making everything worse in this case was Yoast SEO.
4. Today I tried to fix my database with this manual: https://guides.wp-bullet.com/fixing-wp_options-table-missing-unique-column-primary-key/

I already did everything described here: https://www.wpbeginner.com/beginners-guide/beginners-guide-to-troubleshooting-wordpress-errors-step-by-step/
I found that there were empty posts written in my database in the wp_posts table even though I wasn’t able to save them on the “new post” page.

Anyone got an idea?

The page I need help with: [log in to see the link]