Product Version: NetBeans IDE Dev (Build 200804291203)
Java: 1.6.0_05; Java HotSpot(TM) Client VM 10.0-b19
System: Windows XP version 5.1 running on x86; MS932; ja_JP (nb)
Project encodings: UTF-8/Win-31j. See also issue about javadoc 128033 (fixed and verified)

Steps:
1. Create JavaApp using eng name and mbyte chars in the project folder
2. Add XML doc
3. Invoke Generate DTD from the pop-up
4. Select dtd doc then invoke 'Generate Documentation...' from the pop-up
5. Select generated html file and open it using 'View' from the pop-up menu

For the project located in C:\新規フォルダ\JavaApplication8
Firefox opens with the following URL:
file:///C:/%E8%AD%81%EF%BD%B0%E9%9A%95%E4%B8%8A%E3%83%B5%E7%B9%A7%EF%BD%A9%E7%B9%9D%EF%BD%AB%E7%B9%9D%C2%80/JavaApplication8/src/javaapplication8/newXMLDocumentDocumentation.html

Err message is shown:
Firefox can't find the file at /C:/譁ｰ隕上ヵ繧ｩ繝ｫ繝
/JavaApplication8/src/javaapplication8/newXMLDocumentDocumentation.html.

IE also was not able to open html file using this URL.

If the app has mbyte in its name  (in the step1) the URL was generated incorrectly:
App location: C:\新規フォルダ\おい
Firefox started with the following URL:
file:///C:/%E8%AD%81%EF%BD%B0%E9%9A%95%E4%B8%8A%E3%83%B5%E7%B9%A7%EF%BD%A9%E7%B9%9D%EF%BD%AB%E7%B9%9D%C2%80/%E7%B8%BA%E7%BF%AB%EF%BC%9E/src/%E7%B8%BA%E7%BF%AB%EF%BC%9E/newXMLDocumentDocumentation.html
Err message shows incorrect URL (see '>'):
Firefox can't find the file at /C:/譁ｰ隕上ヵ繧ｩ繝ｫ繝/縺翫＞/src/縺翫＞/newXMLDocumentDocumentation.html.

IE also was not able to open html file using this URL.