Follow the steps listed in the Server Reference Manual in regards to installation with IIS. It's not difficult, but it's not as easy as just disable QVWS and enable IIS. You need to do some prepping before.
In case you don't get it, I'd recommend you to do a backup of the C:\ProgramData\QlikTech folder and reinstall your version of QlikView Server setting in the installation process that IIS is the preferred Web Server.
If you already did all the above, go to the QMC and check the System, Setup, QlikView Web Server settings, make sure the paths there are pointing to an existing, accesible folder in your disk.
Hope that helps.
It's been more than 6 months that we've installed and configured QlikView Server according to the manual. From time to time I get the same error in the first post when I try access the application from our Asp.Net application. I manually restart our IIS in order to get this work. What could be the possible reason for this error and how can I avoid this?