Sorry for the delay. I've had to find time to install a fresh sandbox just to test this thing.
Not sure where you got the incorrect filename thing from. The file is named the same as the class.
Anyway, here is what I did to test the above code:
1. Install fresh 2.3.2
2. Install swfupload - dev/build
3. Install dataobject_manager - dev/build
4. Add TwoColumnPage.php and SideBarBlock.php to mysite/code
5. Add SortableDataObject::add_sortable_class('SideBarBlock'); to dataobject_manager/_config.php
6. dev/build - confirm SortOrder is being added to SideBarBlock table.
7. Add new TwoColumnPage page to the CMS. Go to SideBarBlocks tab and add 3 entries.
8. Activate Drag and Drop reordering and reorder items just added.
9. Save and Publish, reload CMS
Problem is exactly the same. The SortOrder is updating just fine in the table, but in the CMS the sorting is only done on the ID (confirmed by manually editing the IDs in the table).
Also, as before, clicking on table column headers does no sorting either. Here is the call the CMS is making when I try to sort on "Header" column: http://silverstripe.localhost/admin/EditForm/field/SideBarBlocks?ctf[SideBarBlocks][per_page]=10&ctf[SideBarBlocks][showall]=0&ctf[SideBarBlocks][sort]=Header&ctf[SideBarBlocks][sort_dir]=ASC&ctf[SideBarBlocks][search]=&ctf[SideBarBlocks][filter]=
Here is the precise files I am using. http://www.zanzomedia.com/mac/code.rar
I can't for the life of me work this out.