I'm having the same issue. When I first launch DCS in 2.5.5 the mouse won't interact with the UI at all. It's not locked up or anything, you just can't interact with it. If I ALT+TAB out and back in, the UI works just fine.
Reading through this thread prompted me to try something. My monitor's native resolution is 2560x1440, but I run DCS at 1920x1080. If I change DCS to run at my monitor's native resolution, the UI works just fine when I launch it. So, it appears to me that this issue is only caused when you're running DCS at a different resolution than your monitor natively supports.