I went my way through the drawing and found that the problem occurs when trying to export a publication ("06 Structure") that has more than one parent.
5/2/2012 5:08:16 PM [Error] Failed to export the element '/ webdav / 06 Structure 1.0 / Building Blocks / System / Page Templates' 5/2/2012 5:08:16 PM [Error] (TCMDAL :: ExecuteRequest: Content Manager returned an error) 5/2/2012 5:08:16 PM [Error] (TCMDAL :: Communications :: CheckResponseForErrors) Errors that occur while checking the response 5/2/2012 5:08:16 PM [Error] (TCMDAL :: Communications :: ExecuteRequest) Error executing the request 5/2/2012 5:08:16 PM [Error] (TCMDAL :: TCMItem :: Read) Error while reading the element '/ webdav / 06 Structure 1.0 / Building Blocks / System / Page Templates'
Parent publication priorities for structure 06:
1. "04 Content Freezer"
2. "04 Content" (note this is also a parent of "04 Content Freezer", if that has any relevance)
3. "04 Design" (note this is the publication that the "Page Templates" items would come from)
I applied hotfix 75095 to the Content Porter server, but the hotfix does not seem to affect this specific issue, although it looks similar conceptually.