Plesk: Cannot start php-fpm failed with code 78 – cannot get uid for user.

A client was trying to increase the file_size limit in php. They were getting this error message.

Error: phpinimng failed: Configuration test '['/opt/plesk/php/8.2/sbin/php-fpm', '--test']' failed with code 78, and message:
[08-Feb-2024 17:53:47] ERROR: [pool xxxxxxxxx] cannot get uid for user 'old_user_name_here'
[08-Feb-2024 17:53:47] ERROR: FPM initilization failed

The cause was; They had changed the username for the “System user’s credentials” in hosting settings. This had not propagated to the php-fpm configuration file in
/opt/plesk/php/8.2/etc/php-fpm.d/domain.com.conf

Running this command did not fix it. Nor did editing the file which is not recommended anyway.
plesk repair web domain.com

The fix was simply a case of going into the php settings and changing it to a different option then back again. For example FastCGI application served by Apache then back again.