User1100352516 posted
Ok, got it.
VS 2017 is parsing the applicationhost.config file before lauching IIS Express.
I've had changed the binding of the project web site to make it reachable from external clients:
<bindings>
<binding protocol="http" bindingInformation="*:49754:*" />
</bindings>
It was working initially without problem, but today suddenly I got the very unhelpful error message....
Changed it to
<bindings>
<binding protocol="http" bindingInformation=":49754:" />
</bindings>
(note: no asterisks!)
now I can reach IIS express from external clients and launch the VS debugger.
HTH
Michael