XF 2.3 Upgrading to R5 from R4

[Aluminum Pints]

... stops with the following error:

RuntimeException: Unable to write to stream in src/vendor/guzzlehttp/psr7/src/Stream.php at line 260

GuzzleHttp\Psr7\Stream->write() in src/vendor/guzzlehttp/psr7/src/StreamDecoratorTrait.php at line 144
GuzzleHttp\Psr7\LazyOpenStream->write() in src/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php at line 414
GuzzleHttp\Handler\CurlFactory::GuzzleHttp\Handler\{closure}()
curl_exec() in src/vendor/guzzlehttp/guzzle/src/Handler/CurlHandler.php at line 44
GuzzleHttp\Handler\CurlHandler->__invoke() in src/vendor/guzzlehttp/guzzle/src/Handler/Proxy.php at line 28
GuzzleHttp\Handler\Proxy::GuzzleHttp\Handler\{closure}() in src/vendor/guzzlehttp/guzzle/src/Handler/Proxy.php at line 48
GuzzleHttp\Handler\Proxy::GuzzleHttp\Handler\{closure}() in src/vendor/guzzlehttp/guzzle/src/PrepareBodyMiddleware.php at line 64
GuzzleHttp\PrepareBodyMiddleware->__invoke() in src/vendor/guzzlehttp/guzzle/src/Middleware.php at line 31
GuzzleHttp\Middleware::GuzzleHttp\{closure}() in src/vendor/guzzlehttp/guzzle/src/RedirectMiddleware.php at line 71
GuzzleHttp\RedirectMiddleware->__invoke() in src/vendor/guzzlehttp/guzzle/src/Middleware.php at line 66
GuzzleHttp\Middleware::GuzzleHttp\{closure}() in src/vendor/guzzlehttp/guzzle/src/HandlerStack.php at line 75
GuzzleHttp\HandlerStack->__invoke() in src/vendor/guzzlehttp/guzzle/src/Client.php at line 333
GuzzleHttp\Client->transfer() in src/vendor/guzzlehttp/guzzle/src/Client.php at line 169
GuzzleHttp\Client->requestAsync() in src/vendor/guzzlehttp/guzzle/src/Client.php at line 189
GuzzleHttp\Client->request() in src/vendor/guzzlehttp/guzzle/src/ClientTrait.php at line 95
GuzzleHttp\Client->post() in src/XF/Service/Upgrade/DownloaderService.php at line 55
XF\Service\Upgrade\DownloaderService->download() in src/XF/Admin/Controller/ToolsController.php at line 535
XF\Admin\Controller\ToolsController->actionUpgradeXf() in src/XF/Mvc/Dispatcher.php at line 358
XF\Mvc\Dispatcher->dispatchClass() in src/XF/Mvc/Dispatcher.php at line 264
XF\Mvc\Dispatcher->dispatchFromMatch() in src/XF/Mvc/Dispatcher.php at line 121
XF\Mvc\Dispatcher->dispatchLoop() in src/XF/Mvc/Dispatcher.php at line 63
XF\Mvc\Dispatcher->run() in src/XF/App.php at line 2702
XF\App->run() in src/XF.php at line 795
XF::runApp() in admin.php at line 15