Another Addendum:
When deactivating CSRF (while having a meta tag for the csrf token and then deleting the meta tag), my server action which was containing the embedded checkout session template has gone completely empty.
Another time it even copied the content of a completely different server action and replaced the server action, where the template for the checkout session was located, with the contents of this other server action.
This is getting out of hand right now.
As soon as I activate CSRF the last Server action I edited gets completely edited and changed to some other server action or is completely empty.
I will have to keep working on my project. It does occur randomly when CSRF is being enabled or deactivated. I cannot replicate it atm. As soon as I have found a way, I will post a video. The first problem with the embedded checkout is still able to be replicated as described.