Version 5.7.3.1
Installing fresh copy. Green lights all across on compatibility.
Gets to the end of installation and gives me a page full of details starting with "grapheme_substr(): grapheme_substr: length is beyond start."

The a big ole stack trace:

Trace:
#0 [internal function]: Whoops\Run->handleError(2, 'grapheme_substr...', 'C:\\wamp\\www\\one...', 197, Array) #1 C:\wamp\www\oneiro\concrete\vendor\patchwork\utf8\class\Patchwork\Utf8.php(197): grapheme_substr('\xC3\xA9\xC3\xA0', 1, -2) #2 C:\wamp\www\oneiro\concrete\src\Utility\Service\Text.php(78): Patchwork\Utf8::substr('welcome', 0, 128) #3 C:\wamp\www\oneiro\concrete\src\Page\Page.php(2548): Concrete\Core\Utility\Service\Text->slugSafeString('welcome') #4 C:\wamp\www\oneiro\concrete\src\Backup\ContentImporter.php(289): Concrete\Core\Page\Page->add(NULL, Array) #5 C:\wamp\www\oneiro\concrete\src\Backup\ContentImporter.php(108): Concrete\Core\Backup\ContentImporter->importPageStructure(Object(SimpleXMLElement)) #6 C:\wamp\www\oneiro\concrete\src\Backup\ContentImporter.php(60): Concrete\Core\Backup\ContentImporter->doImport(Object(SimpleXMLElement)) #7 C:\wamp\www\oneiro\concrete\src\Package\StartingPointPackage.php(97): Concrete\Core\Backup\ContentImporter->importContentFile('C:\\wamp\\www\\one...') #8 [internal function]: Concrete\Core\Package\StartingPointPackage->install_dashboard() #9 C:\wamp\www\oneiro\concrete\controllers\install.php(286): call_user_func(Array) #10 [internal function]: Concrete\Controller\Install->run_routine('elemental_full', 'install_dashboa...') #11 C:\wamp\www\oneiro\concrete\src\Controller\AbstractController.php(156): call_user_func_array(Array, Array) #12 C:\wamp\www\oneiro\concrete\src\Routing\ControllerRouteCallback.php(25): Concrete\Core\Controller\AbstractController->runAction('run_routine', Array) #13 C:\wamp\www\oneiro\concrete\src\Routing\Router.php(59): Concrete\Core\Routing\ControllerRouteCallback->execute(Object(Concrete\Core\Http\Request), Object(Concrete\Core\Routing\Route), Array) #14 C:\wamp\www\oneiro\concrete\src\Support\Facade\Facade.php(116): Concrete\Core\Routing\Router->execute(Object(Concrete\Core\Routing\Route), Array) #15 C:\wamp\www\oneiro\concrete\src\Application\Application.php(342): Concrete\Core\Support\Facade\Facade::__callStatic('execute', Array) #16 C:\wamp\www\oneiro\concrete\src\Application\Application.php(342): Concrete\Core\Support\Facade\Route::execute(Object(Concrete\Core\Routing\Route), Array) #17 C:\wamp\www\oneiro\concrete\bootstrap\start.php(196): Concrete\Core\Application\Application->dispatch(Object(Concrete\Core\Http\Request)) #18 C:\wamp\www\oneiro\concrete\dispatcher.php(36): require('C:\\wamp\\www\\one...') #19 C:\wamp\www\oneiro\index.php(2): require('C:\\wamp\\www\\one...') #20 {main}