I guess that it could have been a number of things, but I looked into my error logs (Ubuntu)
/var/log/apache2/error.log
and saw this error:
PHP Parse error: syntax error, unexpected 'yield' (T_YIELD), expecting '(' in /var/www/path/to/laravel/helpers.php on line 563
so I did a Google search.
There is a Laravel function called yield() that collides with a reserved name in PHP 5.5 that was not in PHP 5.4. It so happens that my old server had PHP 5.4.
Found this which helped me to find all instances of 'yield()' in Laravel and change to '_yield()'. Problem solved!