Please confirm that the assets directory has appropriate permissions for the web server to write.
You can check this on the Security tab when right clicking "assets" and going to Properties. In that Security tab, there should be an IUSR entry listed there which has write permissions.
You can also force IIS to reveal error messages by doing the following:
1. Open Internet Information Services (IIS) Manager
2. Click your server name on the left hand Connections panel
3. Double click the Error Pages icon on the right
4. Click Edit Feature Settings on the right hand sidebar
5. Click Detailed errors in the radio button listing then click OK
6. Restart IIS by going back to the server name in the left hand Connections panel and clicking Restart on the right hand sidebar
This should at least stop the annoying generic IIS errors, and you'll get a detailed error message which should give you some leads.
If you get an error message and are not sure how to solve it, paste it in this thread and we can see what we can do.