The UI occasionally becomes unresponsive. The camera can still be
zoomed and panned, but not by clicking and dragging the mouse. The buttons still show a clicking animation, but nothing happens: none of the menus can be opened, buildings can’t be placed and the time can’t be adjusted. It’s also not possible to switch back into the game from alt-tabbing. The game does continue to run. Autosaves created during the freeze can be loaded and work, but only after force-exiting the game through the task bar.
I have not been able to identify a specific trigger: So far it has happened in the game and in the main menu as well, while waiting and while building. It usually occurs after 5-10 minutes of playing the game, very reliably so.
My computer has Windows 10, with an intel quadcore CPU and NVidia GTX 970 graphics. I will test if it also happens on my other computer, and can send you any further details you might need tomorrow.
Edit: The bug does not seem to occur on my notebook, with Windows 10, an i5-5200U dual core CPU and integrated graphics. I also updated the description above.