I'm betting this may be related to https://jira.duraspace.org/browse/DS-776. I'm not sure the patch will address your problem, but it looks like it might.
>>> On 2/23/2011 at 8:00 PM, in message
> Hi all !. I'm facing a problem when i want to add thumbnail to an item.
> I've added the Admin authority for Editor group for the Bitstream object of
> a collection, but the Editor group still can't upload a thumbnail bundles to
> an item !!
> When i jump to the Authorization setting for an item, there are not
> Policies for Thumbnail Bundles ( but there are policies for original, text,
> licenses bundles ), i don't know what to do now ? Please help.
> Thank you in advance.