This form is not available for this list.

SharePoint lets you create a lookup field in a custom list or document library that reads values off an existing list. If you then create a view with a grouping on the lookup field, then you will get the "This form is not available for this list" error message, if any value in the list is deleted. It does not matter if the value you are linking to still exists, all links via the group by will return the same message.

To work around it, include the lookup field in your view. The field will then appear twice, when expanded, in the grouping as well as in each item, however the links from the grouping will work. However, if you remove the field again, the error will reappear.

Go figure.