The websites on one of my Plesk users can't be accessed. The server reports a 500 Internal server error, the error_log for that user shows a bunch of
[warn] mod_fcgid: read data timeout in 60 seconds
[error] Premature end of script headers: index.php
<?php phpinfo(); ?>
The problem was caused by a rogue
file_get_contents in some scripts.
I looked through the error log for the 1st appearance of the error message, and found a file created exactly when the error message first appeared - only 2 years earlier... WordPress Site hacked? Suspicious PHP file
So I removed the malware ( detailed write-up at https://talk.plesk.com/threads/debugging-premature-end-of-script-headers.338956/ ), rebooted the Server and the error is now gone.
Technical detail: The error turned up because the Server distributing the malware is offline.
file_get_contents("http..." timed out, the local script failed and returned the error message.