problems with loading RDF datasource after using XMLHttpRequest GET

I have the following problem: I get Error: 2152596467 in XMLSinkObserver 
onError when trying to load local rdf file mytmp.rdf with nsIRDFService 
GetDataSource.

This happens only when I have first used XMLhttpRequest http GET to load 
http://www.sample.org/myweb.rdf and saved that to the same local file 
mytmp.rdf. I can see the file and it's content with no problems before 
the datasource load happens in my desktop.

If I load the same file mytmp.rdf as a datasource in a separate session 
I have no problems.

Maybe XMLhttpRequest leaves something open that interferes with 
GetDataSource() but I haven't find any clues of what that might be. I 
even abort the XMLhttpRequest after the loaded content was stored to the 
local file hoping that would close some pipes but it did not help.

Any ideas where to find help?

Marja