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I keep on getting this error on updating a user:

PHP Warning: Cannot modify header information - headers already sent by (output started at /wp-admin/includes/update.php:134) in /wp-includes/pluggable.php on line 866, referer: /wp-admin/user-edit.php?user_id=4&wp_http_referer=%2Fwp-admin%2Fusers.php

Any ideas why?

My header.php does not seem to start with an empty space, but just with <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

And, besides, this seems to be more backend related...

Solution

Somehow pluggable.php or update.php or one of the user edit files got corrupted. After the upgrade to 3.3.2 all good again. All users can be updated again within getting stuck in error reports.

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1 Answer

up vote 1 down vote accepted

It is usually because there are spaces, new lines, or other stuff before an opening tag, typically in wp-config.php. This could be true about some other file too, so please check the error message, as it lists the specific file name where the error occurred (see the link below for more info on "Interpreting the Error Message"). Replacing the faulty file with one from your most recent backup or one from a fresh WordPress download is your best bet, but if neither of those are an option, please this codex: http://codex.wordpress.org/FAQ_Troubleshooting#How_do_I_solve_the_Headers_already_sent_warning_problem.3F

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After the upgrade all was well. Must have been a faulty file as mentioned in the question. I could not answer my own question yesterday. Had to wait for several hours. Thanks for the input! –  rhand Apr 22 '12 at 4:26
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