I just installed the Allvideo3 to use with the latest K2 and now have the same issue which I had with SIG.
1. With the new dim setting on, I can't access the tabs in K2 item section. I disabled that feature in Allvideo and it worked fine. The same issue was with SIG Pro which I fixed by choosing a different display method in the plugin (slimbox-mootools)

2. Also, what ever changes were made in Allvideo3 created issues with the video from provider section.
It says to select a video provider but there is no drop-down menu to choose, just the field for entering the video ID-See attached screenshot.

I get the same issue, except for disabling the dim setting does not fix it. I have to disable the allvideos plugin and then go remove the video on the item to be able to use the tabs with the allvideos plugin enabled.

From another post about allvideo here, I found out that you need to go back to 2.54 until the next release of K2 to use 3.0. If you want to use it now, there is a temporary fix you can add. Search for that post and you will find it.