The problem

I had a custom content-type which was partly indexed. When viewing the content type, parts of the edit form showed raw HTML code instead of widgets:

The solution

I hunted around for a while but checked the contents of the index. Strangely, it had a whole page indexed. After investigating further: it’s another object in the portal with the same id as the attribute on the content-type. The error happens only, if the attribute is missing on the content-type and an object in the hirarchy above has the name id.

So - if you encounter a problem like this, check if the portal_catalog may grab a different object for indexing with the same name of your attribute.