In my case, it was something else.
When I loaded the solution in a new version of Visual Studio, VS apparently created a new project-specific applicationhost.config file:
MySolutionDir\.vs\config\applicationhost.config
It started using the settings from the new config, instead of my already customized global IIS Express settings. (\Users\%USER%\Documents\IISExpress\config\applicationhost.config)
In my case this was the setting that needed to be set. Of course it could be something else for you:
<section name="ipSecurity" overrideModeDefault="Allow" />