If QWebEngineView has loaded a page from single-threaded server, no connections from other clients are accepted until the application with QWebEngineView has been closed. Maybe the tcp connection has not been closed properly. This happens when the server is running on Windows 10 (macOS, Win7 works correctly). The clients can be on any platform.

This is regression, because the error was not reproducible in Qt 5.7. Also the problem is not visible on multi-threaded server.

The problem can be reproduced with Simple Browser example on localhost.
1. Run single-threaded server on Windows 10. Python SimpleHTTPServer can be used for testing:
For python2:
python -m SimpleHTTPServer 8000
For python3:
python -m http.server 8000
2. Run Simple Browser and connect to localhost:8000
3. Run any other browser and try to connect to localhost:8000
4. The second connection fails.
5. Close Simple Browser. The second connection succeeds.

If the repro steps are performed with two Chrome browsers, both connections succeed.