no source error

i'm getting an error i can't debug. the error only shows up in visual web developer (express 2010). i can start the website, browse to a few pages but on one page (only) i get the following error IN vwd:

No Source Available

Call stack location:
AjaxControlToolkit.DLL!AjaxControlToolkit.ExtenderControlBase.OnLoad(System.EventArgs e) Line 306

Source file information:

Locating source for 'f:\TeamCity\buildAgent\work\80acd78aa4c25314\Server\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs'. Checksum: MD5 {77 12 2 51 e9 6d 39 a4 13 34 4d 61 56 3e 88 d7}
The file 'f:\TeamCity\buildAgent\work\80acd78aa4c25314\Server\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs' does not exist.
Looking in script documents for 'f:\TeamCity\buildAgent\work\80acd78aa4c25314\Server\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs'...
Looking in the projects for 'f:\TeamCity\buildAgent\work\80acd78aa4c25314\Server\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs'.
The file was not found in a project.
Looking in directory 'C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\vc7\atlmfc'...
Looking in directory 'C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\vc7\crt'...
Looking in directory 'c:\'...
The debug source files settings for the active solution indicate that the debugger will not ask the user to find the file: f:\TeamCity\buildAgent\work\80acd78aa4c25314\Server\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs.
The debugger could not locate the source file 'f:\TeamCity\buildAgent\work\80acd78aa4c25314\Server\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs'.

sometimes the (dev) web server also errors-out.

i think i have more clues:

it's the only page i'm using a controlbundle on. i removed the bundle and got an error in the browser. i ran a trace and got an 'Unhandled Execution Error' on Begin Load:

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).
  at System.Web.UI.ControlCollection.Add(Control child)
  at AjaxControlToolkit.ScriptObjectBuilder.RegisterCssReferences(Control control) in f:\TeamCity\buildAgent\work\80acd78aa4c25314\Server\AjaxControlToolkit\ExtenderBase\ScriptObjectBuilder.cs:line 323
  at AjaxControlToolkit.ExtenderControlBase.OnLoad(EventArgs e) in f:\TeamCity\buildAgent\work\80acd78aa4c25314\Server\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs:line 305
  at System.Web.UI.Control.LoadRecursive()
  at System.Web.UI.Control.LoadRecursive()
  at System.Web.UI.Control.LoadRecursive()
  at System.Web.UI.Control.LoadRecursive()
  at System.Web.UI.Control.LoadRecursive()
  at System.Web.UI.Control.LoadRecursive()
  at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)