Hello!
In my game I use eventsystem not only for UI, but for the in-game buttons too.
Having 2D Raycaster component on the main camera and Collider2D + EventTrigger on the button makes it working well. On 5.3.
After upgrading to 5.5, only UI buttons work well, but in-game buttons not.
Was there any changes to the Event system in 5.5?