This is the second time I’ve come across this issue due to upgrading Windows, and the usual searching isn’t terribly helpful. With UAC in recent flavors of Windows, you’re required to run Visual Studio as an administrator to debug a website using IIS. This means that when you debug, the application launches as an administrator as well. This is fine 99% of the time, except for when you want to interact with other programs that don’t run as an administrator. Most of the time, you just run the other app as an admin and you’re good to go. Unfortunately file explorer does not have this ability - which is also why dragging and dropping files into Visual Studio no longer works.
You have two options for this. The first is to close the instance of Visual Studio which is running as an admin, and restart it without using Run As Administrator. This first way is by far the easiest fix, but it isn’t always an option. The second way is to completely turn off UAC. The option from the settings charm/control panel is really a “how annoying do you want me to be” meter and doesn’t even come close to killing it, so you have to do it in the registry. Run regedit, navigate to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\System, set EnableLUA to 0, and reboot. This will likely cause issues elsewhere, so you will probably want to turn it back on once you’ve completed debugging.