Yeah, this problem is really annoying.
I am having the same problem.
The related items dropdown doesn't have any products loaded in the list.
When I enter a search phrase in the dropdown, a search application loads quickly, but then vanishes.
Which include or other php file controls this, maybe we can debug.
Even after a fresh install when everything else is working fine, this problem still persists.
Any developers, please advise!
Thanks,
Brian