this is the Error-Code when i try to install eCommerce app in odoo community version on the dashboard on my ubuntu 16.4. server. 

Fehler:
Odoo Server Error

Traceback (most recent call last):
  File "/odoo/odoo-server/odoo/tools/cache.py", line 84, in lookup
    r = d[key]
  File "/odoo/odoo-server/odoo/tools/func.py", line 68, in wrapper
    return func(self, *args, **kwargs)
  File "/odoo/odoo-server/odoo/tools/lru.py", line 44, in __getitem__
    a = self.d[obj].me
KeyError: ('ir.model.data', <function IrModelData.xmlid_lookup at 0x7f9c6300b2f0>, 'base.user_demo')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/odoo/odoo-server/odoo/tools/convert.py", line 744, in parse
    self._tags[rec.tag](rec, de, mode=mode)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 642, in _tag_record
    f_val = self.id_get(f_ref)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 727, in id_get
    res = self.model_id_get(id_str, raise_if_not_found)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 733, in model_id_get
    return self.env['ir.model.data'].xmlid_to_res_model_res_id(id_str, raise_if_not_found=raise_if_not_found)
  File "/odoo/odoo-server/odoo/addons/base/ir/ir_model.py", line 1334, in xmlid_to_res_model_res_id
    return self.xmlid_lookup(xmlid)[1:3]
  File "<decorator-gen-21>", line 2, in xmlid_lookup
  File "/odoo/odoo-server/odoo/tools/cache.py", line 89, in lookup
    value = d[key] = self.method(*args, **kwargs)
  File "/odoo/odoo-server/odoo/addons/base/ir/ir_model.py", line 1323, in xmlid_lookup
    raise ValueError('External ID not found in the system: %s' % xmlid)
ValueError: External ID not found in the system: base.user_demo

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/odoo/odoo-server/odoo/http.py", line 650, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/odoo/odoo-server/odoo/http.py", line 310, in _handle_exception
    raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
  File "/odoo/odoo-server/odoo/tools/pycompat.py", line 87, in reraise
    raise value
  File "/odoo/odoo-server/odoo/http.py", line 692, in dispatch
    result = self._call_function(**self.params)
  File "/odoo/odoo-server/odoo/http.py", line 342, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/odoo/odoo-server/odoo/service/model.py", line 97, in wrapper
    return f(dbname, *args, **kwargs)
  File "/odoo/odoo-server/odoo/http.py", line 335, in checked_call
    result = self.endpoint(*a, **kw)
  File "/odoo/odoo-server/odoo/http.py", line 936, in __call__
    return self.method(*args, **kw)
  File "/odoo/odoo-server/odoo/http.py", line 515, in response_wrap
    response = f(*args, **kw)
  File "/odoo/odoo-server/addons/web/controllers/main.py", line 938, in call_button
    action = self._call_kw(model, method, args, {})
  File "/odoo/odoo-server/addons/web/controllers/main.py", line 926, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/odoo/odoo-server/odoo/api.py", line 689, in call_kw
    return call_kw_multi(method, model, args, kwargs)
  File "/odoo/odoo-server/odoo/api.py", line 680, in call_kw_multi
    result = method(recs, *args, **kwargs)
  File "<decorator-gen-42>", line 2, in button_immediate_install
  File "/odoo/odoo-server/odoo/addons/base/module/module.py", line 71, in check_and_log
    return method(self, *args, **kwargs)
  File "/odoo/odoo-server/odoo/addons/base/module/module.py", line 449, in button_immediate_install
    return self._button_immediate_function(type(self).button_install)
  File "/odoo/odoo-server/odoo/addons/base/module/module.py", line 542, in _button_immediate_function
    modules.registry.Registry.new(self._cr.dbname, update_module=True)
  File "/odoo/odoo-server/odoo/modules/registry.py", line 85, in new
    odoo.modules.load_modules(registry._db, force_demo, status, update_module)
  File "/odoo/odoo-server/odoo/modules/loading.py", line 380, in load_modules
    loaded_modules, update_module, models_to_check)
  File "/odoo/odoo-server/odoo/modules/loading.py", line 274, in load_marked_modules
    perform_checks=perform_checks, models_to_check=models_to_check
  File "/odoo/odoo-server/odoo/modules/loading.py", line 184, in load_module_graph
    _load_data(cr, module_name, idref, mode, kind='demo')
  File "/odoo/odoo-server/odoo/modules/loading.py", line 95, in _load_data
    tools.convert_file(cr, module_name, filename, idref, mode, noupdate, kind, report)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 791, in convert_file
    convert_xml_import(cr, module, fp, idref, mode, noupdate, report)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 852, in convert_xml_import
    obj.parse(doc.getroot(), mode=mode)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 751, in parse
    exc_info[2]
  File "/odoo/odoo-server/odoo/tools/pycompat.py", line 86, in reraise
    raise value.with_traceback(tb)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 744, in parse
    self._tags[rec.tag](rec, de, mode=mode)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 642, in _tag_record
    f_val = self.id_get(f_ref)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 727, in id_get
    res = self.model_id_get(id_str, raise_if_not_found)
  File "/odoo/odoo-server/odoo/tools/convert.py", line 733, in model_id_get
    return self.env['ir.model.data'].xmlid_to_res_model_res_id(id_str, raise_if_not_found=raise_if_not_found)
  File "/odoo/odoo-server/odoo/addons/base/ir/ir_model.py", line 1334, in xmlid_to_res_model_res_id
    return self.xmlid_lookup(xmlid)[1:3]
  File "<decorator-gen-21>", line 2, in xmlid_lookup
  File "/odoo/odoo-server/odoo/tools/cache.py", line 89, in lookup
    value = d[key] = self.method(*args, **kwargs)
  File "/odoo/odoo-server/odoo/addons/base/ir/ir_model.py", line 1323, in xmlid_lookup
    raise ValueError('External ID not found in the system: %s' % xmlid)
odoo.tools.convert.ParseError: "External ID not found in the system: base.user_demo" while parsing /odoo/odoo-server/addons/website_sale/data/demo.xml:789, near
<record id="website_sale_order_1" model="sale.order">
            <field name="partner_id" ref="base.res_partner_3"/>
            <field name="partner_invoice_id" ref="base.res_partner_address_25"/>
            <field name="partner_shipping_id" ref="base.res_partner_address_25"/>
            <field name="user_id" ref="base.user_demo"/>
            <field name="pricelist_id" ref="product.list0"/>
            <field name="team_id" ref="sales_team.salesteam_website_sales"/>
            <field name="date_order" eval="(datetime.now()-relativedelta(days=7)).strftime('%Y-%m-%d %H:%M:%S')"/>
        </record>