From that error message it looks like this plugin is trying to create its own db objects. It should not be doing that. If it wants access to a db, it should ask for it from calibre. For example with
Code:
from calibre.gui2.ui import get_gui
db = get_gui().current_db
to get the currently opened library or for other libraries
Code:
from calibre.gui2.ui import get_gui
library_broker = get_gui().library_broker
newdb = library_broker.get_library(library_path)
Note that using library_broker requires calibre >= 3.0