Yes the change of the default skin to classic has caused a few little issues around the place for some of the plugins.
I know for a lot of ones not included in the plugin_manager need to be manually fixed up, by creating a new skin folder classic. Most of the time you can just copy the old default folder and rename it.
There is more fun when you start going over to the dev version 9, and you have to start replacing old hooks and the removal of rcube_mdb2.php for DB access.