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John is correct. If the avatar file fails to upload, you will have additional problems. The uploading could very well be caused by the issue I referenced above. Fix that, and the upload process may function properly and the other errors go away.

Of course, this is not guaranteed. Let’s just look into this possibility first and see what happens.

Here’s more information of the “Cannot modify header” error: https://codex.wordpress.org/Answers-Troubleshooting#Headers_already_sent