I have Visual Studio 2019 and a very large web application - and get this pop-up Extender error almost every time I open a solution and try to build it from a fresh start on the day. i.e. App opened Friday and compiled fine, Monday it fails - no changes...

The error always references remove and recreate the web.config

FIX is simple -

• open web.config

• press the space-bar

• press backspace
  The web.config is now in a changed state yet no changes were actually made

• Click to run again and it works fine - no actual code changes were made.

I think the problem is due to the complexity of the application, it cannot process the previously complied web.config for some reason that is cached somewhere - changing it's state to changed even though technically it hasn't been changed causes Visual Studio to rebuild it again and it works.