I am using Nortons 360. I use it on everything. I did shut it down to see if I could get Readera to work, but it didn't make a difference
I actually have two Nexus 7 2nd gen tablets. The one tablet that appears to have no issues is my Samsung tab s5e. It syncs on it, but the sync doesn't reach the other tablets because they can't sync.
I do know that there was only one time that the sync worked and when I got on my Samsung tablet and went to Readera and opened the ebook it showed that the book mark on another device showed a different page and asked if I wanted to go to it. I did and it took me to the proper location in the ebook.
I will try uninstalling Nortons and open the security on the Nexus tables and see if that makes a difference.
Let me remind you that it does work on the Samsung and Nortons is installed on it too.