When activating the Help Perspective "The page could not be found" is displayed.
The message is generated in the berryHelpWebView when trying to set a source which is NULL.

This in turn is due to the berryHelpEditor seemingly not finding a home page in the following code:

// an empty url represents the home page

HelpEditorInput::Pointer helpInput = input.Cast<HelpEditorInput>();
QString currHomePage = HelpPluginActivator::getInstance()->getQHelpEngine().homePage();
if (helpInput->GetUrl().isEmpty() && !currHomePage.isEmpty())
{
  helpInput = HelpEditorInput::Pointer(new HelpEditorInput(currHomePage));
}
QtEditorPart::SetInput(helpInput);
m_WebView->setSource(helpInput->GetUrl());

So it seems the bug fix would boil down to setting the home page at the correct position.

one additional issue where this problem appears is when trying to open an actual web url such as on the page "Using The Diffusion Imaging Application" by clicking on "neuroimaging group"

It seems the home page can be set in the workbench advisor

Actually it is set (for the ext app) in
QmitkExtApplicationPlugin.cpp

[0f7b0a]: Merge branch 'bug-11666-help-404'

Merged commits:

2012-04-18 16:00:30 Caspar Goch [60b70e]
Excluded help perspective from list

2012-04-18 15:59:58 Caspar Goch [b4a594]
Switched from mitkPluginActivator to QmitkDiffusionApplicationPlugin

2012-04-18 15:55:51 Caspar Goch [edaaef]
Added needed libraries to provisioning file

2012-04-17 16:41:35 Caspar Goch [8752e9]
Merge remote-tracking branch 'origin/personal/hering/MITKDiffusion-2012-Bugfixes' into bug-11666-help-404

Conflicts:
Plugins/org.mitk.gui.qt.diffusionimagingapp/files.cmake
Plugins/org.mitk.gui.qt.diffusionimagingapp/src/internal/mitkPluginActivator.cpp

2012-04-17 14:39:21 Jan Hering [8d7940]
Merge remote-tracking branch 'origin/bug-11648-SubtractFibsCrash' into personal/hering/MITKDiffusion-2012-Bugfixes

Conflicts:
Plugins/org.mitk.gui.qt.diffusionimaging/src/internal/QmitkFiberProcessingViewControls.ui

2012-04-17 14:36:50 Jan Hering [362805]
Merge remote-tracking branch 'origin/bug-11650-RoiImageCrash' into personal/hering/MITKDiffusion-2012-Bugfixes

2012-04-17 14:36:33 Jan Hering [85ddec]
Merge remote-tracking branch 'origin/bug-11657-tensorToDwiToTensor' into personal/hering/MITKDiffusion-2012-Bugfixes

2012-04-17 14:36:18 Jan Hering [87a2ff]
Merge remote-tracking branch 'origin/bug-11705-GibbsTrackingGui' into personal/hering/MITKDiffusion-2012-Bugfixes

2012-04-17 14:28:20 Caspar Goch [0e44bb]
Changed include directories

2012-04-17 13:55:44 Caspar Goch [800fcf]
Deleting some old commented out code

2012-04-17 13:55:29 Caspar Goch [567664]
WIP: Moving perspectives to own directory

2012-04-17 10:58:22 Caspar Goch [c038f7]
Removed some old debris

2012-04-16 17:32:18 Jan Hering [12e1fa]
Merge remote-tracking branch 'origin/bug-11665-EvalPerspective' into personal/hering/MITKDiffusion-2012-Bugfixes

2012-04-16 17:31:53 Jan Hering [ee4d4e]
Merge remote-tracking branch 'origin/bug-11699-OdfColorCoding' into personal/hering/MITKDiffusion-2012-Bugfixes

Conflicts:
Modules/DiffusionImaging/Rendering/mitkOdfVtkMapper2D.txx

2012-04-16 14:55:55 Caspar Goch [fc4f95]
Merge remote-tracking branch 'origin/bug-11001-move-general-manual-to-ext-plugin' into bug-11666-help-404

2012-04-16 14:18:19 Caspar Goch [9ae1d6]
Merge branch 'bug-11542-fix-mitk-Diffusion-not-starting' into bug-11666-help-404

Closing for now, as it seems it is fixed.