I've upgraded from 4.4 to 5.0, and when trying to edit any product that is a custom product type I get this error:

Server Error in '/' Application.

---

Specified argument was out of the range of valid values.
Parameter name: The view specified in the url cannot be found in the Views collection of the current control.

[ArgumentOutOfRangeException: Specified argument was out of the range of valid values.
Parameter name: The view specified in the url cannot be found in the Views collection of the current control.]
   Telerik.Sitefinity.Web.UI.ContentUI.ContentView.DetermineCurrentViewName() +461
   Telerik.Sitefinity.Web.UI.ContentUI.ContentView.CreateChildControls() +60
   System.Web.UI.Control.EnsureChildControls() +102
   System.Web.UI.Control.PreRenderRecursiveInternal() +42
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2496

Whilst I'm at it, if I try to edit a back end page I get this error:

Could not find the specified key "Breadcrumb" or class id "PageResources".

[ArgumentException: Could not find the specified key "Breadcrumb" or class id "PageResources".]
   Telerik.Sitefinity.Localization.Resource.GetString(ResourceDataProvider provider, String classId, String key, CultureInfo culture, Boolean falback, Boolean throws) +411
   Telerik.Sitefinity.Localization.Res.Get(String classId, String key, CultureInfo culture, Boolean fallback, Boolean throws) +152
   Telerik.Sitefinity.Web.UI.ZoneEditor.CreateToolboxDock(ToolboxItem item) +876
   Telerik.Sitefinity.Web.UI.ZoneEditor.ProcessToolboxSection(ToolboxSection section) +232
   Telerik.Sitefinity.Web.UI.ZoneEditor.CreateToolBoxPanelbar(Toolbox toolbox, String id) +316
   Telerik.Sitefinity.Web.UI.ZoneEditor.CreateControlToolBox() +59
   Telerik.Sitefinity.Web.UI.ZoneEditor.CreateToolbox() +167
   Telerik.Sitefinity.Web.UI.ZoneEditor.CreateChildControls() +144
   System.Web.UI.Control.EnsureChildControls() +102
   System.Web.UI.Control.PreRenderRecursiveInternal() +42
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2496

Hello,

Concerning the error:

Thank you for pointing me in the right direction. The issue was a conflict between the new Sitefinity breadcrumb control and our own breadcrumb control that we had built and installed ourselves. I only had to change the name of our breadcrumb control and that made it all work again.

The first issue (editing products with a custom product type produces error page) appears to be because my CatalogConfig.config is missing all the views and definitions for those product types. I've just created a new product type as a test and it's all working fine. So somehow recently I've lost all my custom product settings in the config files. I'm not sure if it was the 4.4 > 5.0 upgrade, or something going horribly wrong in our systems.

Update: Problem solved. My backup contained most of the relevant settings, and anything missing has been manually recreated. I'm still not sure how it happened, I assume this isn't a recognised upgrade bug?

Must not be because I just upgraded to 5.2 and your post saved me a lot of time. 

Thanks!