In the XML, are all of your uniqueID's unique?
Going from Arena 6 to 7 worked great for me.
Personally, anytime a Major release is pushed out, that is the time of year when I rebuild my comps from scratch. Especially when my original comp is over a year old. Gives me the time to re think my strategy and learn the new features. Might be a good idea for you too if you have the time?
Thank you very much.
In my case, the problem came from the double uniqueid used in two different custom plugins.
during the composition being opened, it seemed that avenue stop the "open process" when first occurence of another plugin with same uniqueid appeared.