Having created a custom content type in schema.xml, when trying to create a list with the content type, I get a message like "path is too long" in the browser. ( HRESULT=0x80010135 )
The problem seemed to be related to either serialization of types in the SharePoint RPC, or problem with reference of types or variables in RPC.
I was following the article "How to: Create a Custom List Definition" (
http://msdn.microsoft.com/en-us/library/ms466023.aspx ) and I used guidgen to generate a new GUID for my content type. I just copied and pasted the GUID, so I had something like
<ContentType ID="C9FC944F-4124-4DB2-9E37-20CD2E55DCF9" ...
That caused the problem. I needed to follow the format described here for content type IDs. To start with, it was missing the leading "0x" to denote hex encoding, and there are no dashes.
Following the content type ID format solved the problem.