Calibre content server crashes

I started the calibre content server from preferences on port 9090 with a custom username and password. When
I try to access it, either from a web browser via the test server option or using Calibre Companion I get a 500 server error.
There is this traceback in the calibre logs:

[26/Feb/2015:17:12:33] HTTP Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cprequest.py", line 670, in respond
    response.body = self.handler()
  File "/usr/lib/python2.7/dist-packages/cherrypy/lib/encoding.py", line 217, in __call__
    self.body = self.oldhandler(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cpdispatch.py", line 61, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "/usr/lib/calibre/calibre/library/server/utils.py", line 54, in do
    ans = func(*args, **kwargs)
  File "/usr/lib/calibre/calibre/library/server/utils.py", line 108, in authenticate
    self.secret)
  File "/usr/lib/python2.7/dist-packages/cherrypy/lib/auth_digest.py", line 364, in digest_auth
    ha1 = get_ha1(realm, auth.username)
  File "/usr/lib/python2.7/dist-packages/cherrypy/lib/auth_digest.py", line 57, in get_ha1
    return md5_hex('%s:%s:%s' % (username, realm, password))
  File "/usr/lib/python2.7/dist-packages/cherrypy/lib/auth_digest.py", line 30, in <lambda>
    md5_hex = lambda s: md5(ntob(s)).hexdigest()
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cpcompat.py", line 68, in ntob
    assert_native(n)
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cpcompat.py", line 111, in assert_native
    raise TypeError("n must be a native str (got %s)" % type(n).__name__)
TypeError: n must be a native str (got unicode)

[26/Feb/2015:17:12:54] HTTP Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cprequest.py", line 670, in respond
    response.body = self.handler()
  File "/usr/lib/python2.7/dist-packages/cherrypy/lib/encoding.py", line 217, in __call__
    self.body = self.oldhandler(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cpdispatch.py", line 61, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "/usr/lib/calibre/calibre/library/server/utils.py", line 54, in do
    ans = func(*args, **kwargs)
  File "/usr/lib/calibre/calibre/library/server/utils.py", line 108, in authenticate
    self.secret)
  File "/usr/lib/python2.7/dist-packages/cherrypy/lib/auth_digest.py", line 364, in digest_auth
    ha1 = get_ha1(realm, auth.username)
  File "/usr/lib/python2.7/dist-packages/cherrypy/lib/auth_digest.py", line 57, in get_ha1
    return md5_hex('%s:%s:%s' % (username, realm, password))
  File "/usr/lib/python2.7/dist-packages/cherrypy/lib/auth_digest.py", line 30, in <lambda>
    md5_hex = lambda s: md5(ntob(s)).hexdigest()
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cpcompat.py", line 68, in ntob
    assert_native(n)
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cpcompat.py", line 111, in assert_native
    raise TypeError("n must be a native str (got %s)" % type(n).__name__)
TypeError: n must be a native str (got unicode)

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: calibre 2.5.0+dfsg-1
ProcVersionSignature: Ubuntu 3.16.0-24.32-generic 3.16.4
Uname: Linux 3.16.0-24-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.14.7-0ubuntu8.2
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Feb 26 19:00:30 2015
InstallationDate: Installed on 2014-11-20 (98 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/zsh
SourcePackage: calibre
UpgradeStatus: No upgrade log present (probably fresh install)