There appears to be a title tag that appears with that error message:

'concrete5 is not compatible with opcode caches that strip PHP comments. Certain configurations of eAccelerator and Zend opcode caching may use this behavior, and it must be disabled.'

So it looks like your server is configured with some kind of PHP cache that doesn't support comments - the comments can't be stripped from concrete5's php code as they are used to set up database structures (in a system called Doctrine)

You'll need to investigate whether you have control over theses kinds of loaded modules. You could try changing your PHP version (to PHP7) if you have that available too.
If you have cPanel, there is normally a section within the PHP selector page where you can check and uncheck modules to load for PHP - perhaps you're running an older version of PHP with eAccelerator enabled.

Thanks for the quick response. Actually I have now found the problem. I looked through the php.ini file and at the bottom I found some eAccelerator stuff. I ; commented it out. Restarted the system and got all green ticks. Have now installed Concrete with the demo input as a refresher, but now I am getting a lot of missing file errors. I will do a new installation without any data and keep my fingers crossed.

Hi Simmo,

On an unrelated note, I recommend using the latest version of concrete5 5.7. Version 5.7.3.1 is fairly old and does not include many of the bug fixes and new features that have been added.