I ran into the same bug. The problem seems to be with units levelling up. Often when a unit levels up, the idle worker count goes up.
I'm guessing the unit gets replaced with an upgraded version, but the old version isn't completely disposed.
I think the reason you can't assign units when this happens, is the game is trying to assign a unit that doesn't exist.
The steps to reproduce seem to be to let a bunch of units level up, and then try and move them around.
Hope that helps
Thanks for the info. I noticed that HUD too. Working on a handful fixes now. The HUD bug from level up was a simple issue where new creatures were auto assigned to follower list and also removed without updating the HUD. No big deal.
The big game breaking bug looks to be an issue where the lists of creatures are not updating properly, and creatures that should be followers are assigned to an object, or creatures that should be assigned to an object are listed as followers.
An easy way to recreate the problem is to rapidly whistle a group of creatures while clicking a crystal or training bag.
