I'm unsure about the severity, reading debian policy it might "only"
be important, please correct severity if I'm mistaken.

debian/copyright is outdated and incomplete.

Example:
sj-cell-renderer-text.c: * Copyright (C) 2014 Phillip Wood <phillip.wood@dunelm.org.uk>
however, Phillip Wood is not in debian/copyright

debian/copyright says: ... debianized ... 2013
however, the latest year is 2008.

The latest upstream release in Debian is from 2014
however, the latest year is 2008.

I did not continue the investigation.