Index: lib/redmine/codeset_util.rb
===================================================================
--- lib/redmine/codeset_util.rb	(revision 20033)
+++ lib/redmine/codeset_util.rb	(working copy)
@@ -23,9 +23,14 @@
       end
       enc = encoding.blank? ? "UTF-8" : encoding
       if enc.casecmp("UTF-8") != 0
-        str.force_encoding(enc)
-        str = str.encode("UTF-8", :invalid => :replace,
+        begin
+          str.force_encoding(enc)
+          str = str.encode("UTF-8", :invalid => :replace,
               :undef => :replace, :replace => '?')
+        rescue Encoding::ConverterNotFoundError
+          Rails.logger.warn ">>> ConverterNotFoundError (#{enc} to UTF-8)"
+          str = replace_invalid_utf8(str)
+        end
       else
         str = replace_invalid_utf8(str)
       end
Index: test/unit/lib/redmine/codeset_util_test.rb
===================================================================
--- test/unit/lib/redmine/codeset_util_test.rb	(revision 20033)
+++ test/unit/lib/redmine/codeset_util_test.rb	(working copy)
@@ -101,4 +101,12 @@
     assert_equal "UTF-8", s2.encoding.to_s
     assert_equal 'こんにち?', s2
   end
+
+  test "#to_utf8 should support not supported code page" do
+    s1 = (+'không hợp lệ').force_encoding('Windows-1258')
+    s2 = Redmine::CodesetUtil.to_utf8(s1, 'Windows-1258')
+    assert s2.valid_encoding?
+    assert_equal 'UTF-8', s2.encoding.to_s
+    assert_equal 'không hợp lệ', s2
+  end
 end