Enabling CustomErrors in web.config leads to WebResource.axd not loead (MIME-Type) and Javascript Syntax Error

Hi all,
When i switch customErrors mode to "on" the browser notifies me with the foloowing information:
Fehler: Stylesheet http://localhost:4350/Error.aspx?aspxerrorpath=/WebResource.axd wurde nicht geladen, weil sein MIME-Typ, "text/html", nicht "text/css" ist.
Quelldatei: http://localhost:4350/default.aspx
Zeile: 0
Fehler: syntax error
Quelldatei: http://localhost:4350/Error.aspx?aspxerrorpath=/WebResource.axd
Zeile: 3
Quelltext:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Which basically means that WebResource.axd could not be loaded cause MIME-Type is not "text/html" or "text/css"
When i turn customErrors "off" the browser is not complaining anymore?
How can i fix this?
thx!

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