We encountered a bug due to usage of German umlaut in Filenames and Folders (MCImageManger and MCFileManager). And we found a Solution!

ASP.NET throws a FileNotFound Exception, because the encoding of the query string is wrong. (ü -> %FC -> is encoded to %ufff)

Plugins\imagemanager\stream\default.aspx: Line: 57 
man.DispatchEvent(EventType.Stream, cmd, Request.QueryString);

The original query string captured by Fiddler results in a HTTP 500 Error.

GET /tinymce/plugins/imagemanager/stream/default.aspx?cmd=im.thumb&path=%7B0%7D/cms/Animation-f%FCr-web.gif&u=99676 HTTP/1.1

If we change the query string in Fiddler, (%FC -> %C3%BC) we get a HTTP 200 OK from our Webserver.

GET /tinymce/plugins/imagemanager/stream/default.aspx?cmd=im.thumb&path=%7B0%7D/cms/Animation-f%C3%BCr-web.gif&u=99676 HTTP/1.1

If we change the JS encoding function from 'escape' to 'encodeURIComponents’, the thumbnail generator works correctly.

//old
thumburl : '../../stream/default.aspx?cmd=im.thumb&path=' + escape(r.path) + '&u=' + r.size,
//new
thumburl : '../../stream/default.aspx?cmd=im.thumb&path=' + encodeURIComponent (r.path) + '&u=' + r.size,

Microsoft also recommends using "encodeURIComponent" with ASP.NET.

Could you please implement an entry in your configuration, so that it’s possible to switch between the different encodings?