There are conflicting installation instructions for BE 2.5. If you read the installation document, it says...
No really, these applications have pools. Must be rich or something. Anyway, if you run with the default IIS7 settings, they'll get nice new shiny pools which don't work with this software. When you convert the BlogEngine folder into an "application"
(what is one of those anyway?), don't just leave the "Application pool" setting as the default.Click the "Select" button and the choose "Classic .NET AppPool" from the dropdown. If you don't do this, some of the software will
fail in strange ways. Trust me on this, at least it's true for 1.0 and 1.1.
But if you read the BE 2.5 upgrade document, it says...
The configuration section 'system.web.extensions' cannot be read because it is missing a section declaration
The likely cause for this is because your website is running ASP.NET 2.0 or 3.5. BlogEngine.NET 2.5 requires your website run in an ASP.NET v4.0 application pool -- ideally in the Integrated mode pipeline, but Classic should work too.
It may be wise to synchronize.