Yes, of course we test on Windows 10. Unfortunately the web browser uses an embedded IE ActiveX component under the hood so the problem may be because you have an old installation of IE that needs to be updated, there is some information in this thread for how to update it.
If it’s still not working, you can compile the Projucer in GPL mode by enabling the JUCER_ENABLE_GPL_MODE in Projucer/Juce Library Code/AppConfig.h and then you won’t need to log in to use the application.