Did you run a /db/build after adding the sortable class? it needs to add a SortOrder column to the object. My guess is after you sort, the AJAX request is failing because it can't find that column.

And needless to say, when you display on the frontend, don't have an order clause in your query. It will order by SortOrder by default.

I have a subdirectory in my localhost, something like http://localhost/silverstripe_2_3_0/.
In silverstripe 2.3.0 rc3 some files aren't found. So i had do delete the leading slashes of directories. The director is forced to take the whole path do build an absolute url.
These files are affected:
facebox.js (Line82): loadingImage : 'dataobject_manager/images/loading.gif',
facebox.js (Line83): closeImage : 'dataobject_manager/images/closelabel.gif',
dataobject_manager.js (Line70): $j.post('DataObjectManager_Controller/dosort/' + do_class, $list.sortable("serialize"));

swfupload/_config.php (Line35): 'flash_url' => Director::absoluteURL('swfupload/javascript/swfupload.swf'),
swfupload/_config.php (Line45): 'button_image_url' => Director::absoluteURL('swfupload/images/XPButtonNoText_160x22.png'),

In css-Files replace /dataobject_manager/images with ../images

I have another problem with sorting the data. I have a page with two dataobject_manager objects. Each of them has a sortable property.
Sorting the data works now in the first tab on list & grid-view, on the second tab, i can only sort the data in grid view, not in list view. I can change the order of the tabs and i have the same effect with the second tab.
Do you have any suggestion?

Okay, I've confirmed it. This is very bizarre. For some reason, only that checkbox has the strange behavior of binding to the first DOM on the page. All the other event handlers are aware of their unique containers. So strange.

Yup. You'll see someone mentioned that a few posts ago. I'll be checking in a new version with the correct pathing soon.