- "ASP.NET Ajax client-side framework failed to load."
- "Sys is not defined"
After some fiddling around with Web.Config handlers and initialization of the ScriptManager, and with no luck, we turned the focus to IIS. After all, these kind of errors might have to do with version problems we thought. And we were right!
A simple switch from .NET Framework v2.0 to v4.0 for the App pool solved it.
Lesson learned: make sure you use the correct .NET version for your website.