I have, to no avail. I've reverted every local GPO policy I could find, and removed any registry entries. I've gone through the list one by one and reverting anything that remotely looks like it might affect thumbnail icons. I think the Optimization Tool makes some changes that are not documented, because when I do a full "undo" of all changes, it restored the thumbnails. In the end, it was quicker to just issue the user a new VDI.

And to whomever develops the Optimization Tool: Please add the ability to undo individual settings!