Strange error running phpunit on CircleCI after upgrading to Laravel 5.4



After an upgrade of Laravel there are now PHP-errors on CircleCI executing tests with phpunit
But on the local dev-environment (PHP 7.013) the test are OK, also on SemaphoreCI (PHP 7.019) and Codeship (PHP 7.019)

The stack:
PHP 7.0, Laravel 5.4, MySQL, Ubuntu

The exact error message:
PHP Fatal error: Cannot use Site as Site because the name is already in use in /home/ubuntu/ on line 14

The above mention line is just
use Site;

Which troubleshooting steps already taken:
Disable XDebug

Any recent changes either in the codebase or the stack:
the app was migrated from Laravel 4.2 to 5.4

What happens when you run the tests locally
run phpunit locally is ok

What is supposed to happen when your build runs
php should run the tests

The build URL if the project is public
the project is not public