At least one person found that having a specific program running in the background (used to allow control of multiple networked computers with the same keyboard and mouse) caused the mouse to be extremely sensitive controlling the camera, etc.

Mouse cursor problems have been associated with performance problems in the game (Game is stuttering, also the cursor in the menu). I'm pretty sure I've seen suggestions for just the mouse issue somewhere, but I'm not finding anything in a search.

Did you try checking the 'don't use desktop resolution' option for the UI?

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First Aid Kit for Divinity II

First Aid : when Div2 does not start or crashes