Synchronization browser does not load ASP.NET 5 website using proxy server

Question

Browsersync seems to be stuck loading the website when starting in proxy mode. Using the same configuration as on the other website, it works, so it should be something in my local setup, but I could not figure out what.

I run it on Windows, proxying an ASP.NET 5 application that runs on localhost: 5000. Directly navigating to this location works fine. Trying to connect to the pipeline using ASP.NET signals, html is exported via the pipeline, but the browser never gets a response and remains on hold.

Logging does not indicate a difference, except for a different session and the obvious proxy server URL, if I run it on a different website, where other sites on the local host (in IIS) also work.

Used configuration (gulp):

gulp.task('browsersync', function () { browserSync({ proxy: 'localhost:5000', notify: true, open: true, logLevel: 'debug' }); });