I've created a very simple custom module to add a boolean field to the hr_expense_line as follows:

__init__.py:

import ocl_hr_expense

__openerp__.py:

{
    'name': "OCL HR Expense Additions",
    'version': "1.0",
    'author': "Paul Strinati",
    'category': "Tools",
    'depends': ['hr'],
    'data': ['ocl_hr_expense.xml'],
    'demo': [],
    'installable': True,
}

ocl_hr_expense.py:

from openerp.osv import fields, osv
class ocl_hr_expense(osv.osv):
    _inherit = "hr.expense.line"
    _columns = {
        'abc': fields.boolean('ABC Reportable')
    }
    _defaults ={
        'abc': 0
    }
ocl_hr_expense()

ocl_hr_expense.xml:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
  <data>
    <record model="ir.ui.view" id="ocl_hr_expense">
      <field name="name">hr.expense.line.tree</field>
      <field name="model">hr.expense.expense</field>
      <field name="inherit_id" ref="hr_expense.view_expenses_form" />
      <field name="arch" type="xml">
        <xpath expr="/form/sheet/notebook/page[@string='Description']/field[@name='line_ids']/tree[@string='Expense Lines']/field[@name='name']" position="after">
          <field name="abc" />
        </xpath>
      </field>
    </record>
  </data>
</openerp>

So far...so good!

The above was done in a test environment called ERP_TEST_IT2 (the DB is duplicated from ERP_LIVE, our live DB, but running off the same filesystem files). Before making any more changes, I want to duplicate this test environment to ERP_TEST_IT3 - I did so via the Manage Databases link on the login page, and having successully supplied the Master password I can see that the new DB instance has been created (and I can see the boolean field in hr_expense_line, as expected).

However, when I try to login to the new ERP_TEST_IT3 environment, I get the following error message:

Client Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/openerp/addons/web/http.py", line 204, in dispatch
    response["result"] = method(self, **self.params)
  File "/usr/lib/pymodules/python2.7/openerp/addons/web/controllers/main.py", line 864, in authenticate
    req.session.authenticate(db, login, password, env)
  File "/usr/lib/pymodules/python2.7/openerp/addons/web/session.py", line 115, in authenticate
    uid = self.proxy('common').authenticate(db, login, password, env)
  File "/usr/lib/pymodules/python2.7/openerp/addons/web/session.py", line 30, in proxy_method
    result = self.session.send(self.service_name, method, *args)
  File "/usr/lib/pymodules/python2.7/openerp/addons/web/session.py", line 103, in send
    raise xmlrpclib.Fault(openerp.tools.ustr(e), formatted_info)

Server Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/openerp/addons/web/session.py", line 89, in send
    return openerp.netsvc.dispatch_rpc(service_name, method, args)
  File "/usr/lib/pymodules/python2.7/openerp/netsvc.py", line 296, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
  File "/usr/lib/pymodules/python2.7/openerp/service/web_services.py", line 433, in dispatch
    return fn(*params)
  File "/usr/lib/pymodules/python2.7/openerp/service/web_services.py", line 444, in exp_authenticate
    res_users = pooler.get_pool(db).get('res.users')
  File "/usr/lib/pymodules/python2.7/openerp/pooler.py", line 49, in get_pool
    return get_db_and_pool(db_name, force_demo, status, update_module)[1]
  File "/usr/lib/pymodules/python2.7/openerp/pooler.py", line 33, in get_db_and_pool
    registry = RegistryManager.get(db_name, force_demo, status, update_module)
  File "/usr/lib/pymodules/python2.7/openerp/modules/registry.py", line 203, in get
    update_module)
  File "/usr/lib/pymodules/python2.7/openerp/modules/registry.py", line 233, in new
    openerp.modules.load_modules(registry.db, force_demo, status, update_module)
  File "/usr/lib/pymodules/python2.7/openerp/modules/loading.py", line 350, in load_modules
    force, status, report, loaded_modules, update_module)
  File "/usr/lib/pymodules/python2.7/openerp/modules/loading.py", line 256, in load_marked_modules
    loaded, processed = load_module_graph(cr, graph, progressdict, report=report, skip_modules=loaded_modules, perform_checks=perform_checks)
  File "/usr/lib/pymodules/python2.7/openerp/modules/loading.py", line 161, in load_module_graph
    models = pool.load(cr, package)
  File "/usr/lib/pymodules/python2.7/openerp/modules/registry.py", line 118, in load
    model = cls.create_instance(self, cr)
  File "/usr/lib/pymodules/python2.7/openerp/osv/orm.py", line 883, in create_instance
    'You may need to add a dependency on the parent class\' module.' % (name, parent_name))
TypeError: The model "hr.expense.line" specifies an unexisting parent class "hr.expense.line"
You may need to add a dependency on the parent class' module.

I then get "Invalid username or password".

Flicking back to ERP_TEST_IT2 I can log in fine.

Is there a trick to duplicating a DB with custom modules?

I've seen posts for similar errors, although not related to having duplicated the DB:

https://www.odoo.com/forum/help-1/question/typeerror-the-model-fleet-vehicle-specifies-an-unexisting-parent-class-fleet-vehicle-17854

and

https://www.odoo.com/forum/help-1/question/getting-typeerror-the-model-hr-contract-specifies-an-unexisting-parent-class-hr-contract-53061

but unfortunately no answers.

Any pointers greatly appreciated, as I've tried deleting and re-duplicating the DB but to no avail.