Server 500 error "end of output script before header"

I keep getting errors using Theshop (1.10) with Woocommerce 3.5.2

users are repeatedly dumped - see log:
[Sat Dec 01 00:56:01 2018] [error] [client 180.190.182.145:0] End of script output before headers: admin-ajax.php
[Sat Dec 01 00:56:01 2018] [error] [client 180.190.182.145:0] End of script output before headers: post.php
[Sat Dec 01 01:07:02 2018] [error] [client 180.190.182.145:0] End of script output before headers: admin-ajax.php
[Sat Dec 01 01:35:01 2018] [error] [client 110.54.240.148:0] End of script output before headers: index.php
[Fri Nov 30 11:06:42 2018] [error] [client 185.151.28.62:0] Script timed out before returning headers: wp-cron.php
[Sat Dec 01 06:11:01 2018] [error] [client 180.190.182.145:0] End of script output before headers: admin-ajax.php
[Sat Dec 01 06:43:01 2018] [error] [client 180.190.182.145:0] End of script output before headers: admin-ajax.php
www.mikesimportedenglishgoods.com [Sat Dec 01 07:00:01 2018] [error] [client 180.190.182.145:0] End of script output before headers: admin-ajax.php

Any ideas/suggestions?
I have tried turning off all plugins (apart from Woocommerce) and the error is still there. The status looks OK PHP 7.2 - plenty of memory etc. FYI, it happens on anther site too. Short of changing theme, I am at a loss. Thanks

Hello, please check this tutorial, it might help:

Please feel free to ask any other questions that you might have.

Kind Regards, Roman.