I'm following the video "View Item Slot Container". I'm using UCC and when I open the panel the player input stops as expected however when the panel closes all mouse input stops and the player can't free look. Only the keyboard input works.
You may have an issue with how your Unity Input or DisplayPanelManagerHandler component are setup. Could you check that they are setup like the screenshots showed under the "Custom Inventory Item Actions" section of the integration documenation page:
The character controller is seamlessly integrated with the Ultimate Inventory System allowing you to use the Ultimate Inventory System inventory rather than the inventory included . . .
opsive.com
(It's around the end)
You can also compare your setup with the one from the integration demo scene