Any idea what is causing this error and how I fix it please?

[25-Apr-2019 09:47:35 UTC] Request URI: /~MYHOSTING/index.php?main_page=page&id=5&chapter=1, IP address: 00.00.000.000
#1 require(/home/MYHOSTING/public_html/includes/init_includes/init_header.php) called at [/home/MYHOSTING/public_html/includes/autoload_func.php:48]
#2 require(/home/MYHOSTING/public_html/includes/autoload_func.php) called at [/home/MYHOSTING/public_html/includes/application_top.php:170]
#3 require(/home/MYHOSTING/public_html/includes/application_top.php) called at [/home/MYHOSTING/public_html/index.php:26]
[25-Apr-2019 09:47:35 UTC] PHP Warning: Use of undefined constant MODULE_PAYMENT_PAYPAL_TESTING - assumed 'MODULE_PAYMENT_PAYPAL_TESTING' (this will throw an Error in a future version of PHP) in /home/MYHOSTING/public_html/includes/init_includes/init_header.php on line 27

The error happened after upgrading from 1.5.5e to 1.5.6a

I have tried replacing all of those files too.