When trying to browse server for flash- and mediafiles I get the following error:
Precondition Failed
The precondition on the request for the URL /FCKeditor/editor/filemanager/browser/default/browser.html evaluated to false.
Can somebody please help?
Precondition Failed
The precondition on the request for the URL /FCKeditor/editor/filemanager/browser/default/browser.html evaluated to false.
Can somebody please help?

Re: "Browse server" returns error
In the config.aspx usually found in /fckeditor/editor/filemanager/connectors/aspx look for the function CheckAuthentication().
By default it returns false. Just read what's in there and take appropriate action.
Returning true will make the error go away, but as stated in the comments, do not just set it to true!
Re: "Browse server" returns error
Re: "Browse server" returns error