I think the problem is that your site is running a legacy version of the post-stream, because discourse-nationalflags has not yet been updated for the new post stream. As an admin, you should be seeing a red banner about that issue.
The recent fixes for user-field-prompt were built only with the new post-stream in mind, so that’s why it’s causing errors on your site.
So if you need this user-field feature right now, I’d recommend disabling the nationalflags plugin. Otherwise, you’ll need to wait for the developer of that plugin to make the necessary updates.
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