ubuntu 14.04を使ってます。
dropboxが起動しなくなりました。
nautilus-dropbox 1.6.1-1

$ dropbox start
Starting Dropbox...Traceback (most recent call last):
  File "dropbox/client/main.py", line 37, in <module>
  File "dropbox/client/aggregation.py", line 17, in <module>
  File "dropbox/sqlite3_helpers.py", line 21, in <module>
  File "dropbox/fatal_db_exception.py", line 1, in <module>
  File "arch/__init__.py", line 28, in <module>
  File "arch/linux/util.py", line 52, in <module>
  File "dropbox/url_info.py", line 19, in <module>
  File "dropbox/i18n.py", line 31, in <module>
  File "babel/__init__.py", line 33, in <module>
  File "pkg_resources/__init__.py", line 3032, in <module>
  File "pkg_resources/__init__.py", line 933, in subscribe
  File "pkg_resources/__init__.py", line 3032, in <lambda>
  File "pkg_resources/__init__.py", line 2573, in activate
  File "pkg_resources/__init__.py", line 2667, in insert_on
  File "pkg_resources/__init__.py", line 2697, in check_version_conflict
  File "pkg_resources/__init__.py", line 2566, in _get_metadata
  File "pkg_resources/__init__.py", line 1589, in get_metadata_lines
  File "pkg_resources/__init__.py", line 1581, in get_metadata
  File "pkg_resources/__init__.py", line 1692, in _get
IOError: [Errno 13] Permission denied: '/var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86-3.6.7/futures-2.1.3-py2.7.egg/EGG-INFO/top_level.txt'

ということでIOError　に関して検索して
http://askubuntu.com/questions/633824/when-starting-dropbox-ioerror-errno-13-permission-denied
に辿り着き

$ sudo chmod +r /var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86-3.6.7/futures-2.1.3-py2.7.egg/EGG-INFO/top_level.txt

としたら dropbox起動出来ました。
同じようなことが起こってたらと思い、書き込みました。参考まで。