++api++/:null is displayed and cannot be operated

I set up Plone 6 using Docker. However, I am having problems changing settings and saving the site at all.
I checked using Chrome's DevTools and got the following error message.

For example, if I try to change the language in SITE SETUP and press the save button in the upper left corner, I get the following error message.

I have been struggling with this issue for over a week. I have checked the past logs of this community and cannot find a solution.
I would very much appreciate your advice.
Best regards.

This might help Nginx and Docker issue - #5 by mikemets

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Thank you very much. I will refer to it. If I don't understand, please let me ask again.