log_threshold > 0 hangs on 1and1 hosted webserver

Hi all,
I hope you can help this new boy?
I am trying to run up a CI application on a 1and1 Linux host. I have it running fine on my local Ubuntu XAMPP environment.

To test, I have put up CI 3.1.6 on the hosted service but it is behaving very oddly.

If I configure log_threshold to anything greater than 0 then the "Welcome to CodeIgniter" just "hangs" - as far as I can tell, nothing is returned to the browser and eventually there seem to be some timeout which get reported as ERR_INCOMPLETE_CHUNKED_ENCODING.

What is more confusing is that reverting log_threshold = 0 does not, immediately, restore the test to a working condition. I am really not sure at what point it starts working again. 

Any thoughts on why log_threshold might not work?
Any thoughts on why reverting the change does not take immediate effect?

FYI: I think you can run phpinfo on this test at http://jakitservices.co.uk/phpinfo.php
(The host supplied PHP as 7.0.24).

In this environment, .htaccess is not required when it works.

Many thanks for any assistance,
 - jon