Hi James, I had this recently just after an upgrade.
The problem seemed to be blank lines at the end of the main config.php file in the moodle root directory.
It may also be blank lines in your theme/yourcustomtheme/config.php.
You can use the configcheck.php script at https://webwork2.math.ohio-state.edu/configcheck.php to check for this problem.
Hope this helps.
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