Odoo 12 CE - Ubuntu

All of a sudden, the scheduled action for the \contract module (Generate Recurring Invoices from Contracts) which calls: 

model.cron_recurring_create_invoice()

Is throwing the following error:

Error:
Odoo Server Error

Traceback (most recent call last):
  File "/opt/odoo12/odoo/odoo/tools/safe_eval.py", line 352, in safe_eval
    return unsafe_eval(c, globals_dict, locals_dict)
  File "", line 1, in <module>
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract.py", line 633, in cron_recurring_create_invoice
    invoices |= contracts_to_invoice._recurring_create_invoice(date_ref)
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract.py", line 618, in _recurring_create_invoice
    invoices_values = self._prepare_recurring_invoices_values(date_ref)
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract.py", line 587, in _prepare_recurring_invoices_values
    invoice_values=invoice_values,
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract_line.py", line 698, in _prepare_invoice_line
    name = self._insert_markers(dates[0], dates[1])
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract_line.py", line 747, in _insert_markers
    name = name.replace('#END#', last_date_invoiced.strftime(date_format))
AttributeError: 'bool' object has no attribute 'strftime'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/odoo12/odoo/odoo/http.py", line 656, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/opt/odoo12/odoo/odoo/http.py", line 314, in _handle_exception
    raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
  File "/opt/odoo12/odoo/odoo/tools/pycompat.py", line 87, in reraise
    raise value
  File "/opt/odoo12/odoo/odoo/http.py", line 698, in dispatch
    result = self._call_function(**self.params)
  File "/opt/odoo12/odoo/odoo/http.py", line 346, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/opt/odoo12/odoo/odoo/service/model.py", line 98, in wrapper
    return f(dbname, *args, **kwargs)
  File "/opt/odoo12/odoo/odoo/http.py", line 339, in checked_call
    result = self.endpoint(*a, **kw)
  File "/opt/odoo12/odoo/odoo/http.py", line 941, in __call__
    return self.method(*args, **kw)
  File "/opt/odoo12/odoo/odoo/http.py", line 519, in response_wrap
    response = f(*args, **kw)
  File "/opt/odoo12/odoo/addons/web/controllers/main.py", line 967, in call_button
    action = self._call_kw(model, method, args, {})
  File "/opt/odoo12/odoo/addons/web/controllers/main.py", line 955, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/opt/odoo12/odoo/odoo/api.py", line 759, in call_kw
    return _call_kw_multi(method, model, args, kwargs)
  File "/opt/odoo12/odoo/odoo/api.py", line 746, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "/opt/odoo12/odoo/odoo/addons/base/models/ir_cron.py", line 83, in method_direct_trigger
    self.sudo(user=cron.user_id.id).ir_actions_server_id.run()
  File "/opt/odoo12/odoo/odoo/addons/base/models/ir_actions.py", line 543, in run
    res = func(action, eval_context=eval_context)
  File "/opt/odoo12/odoo/addons/website/models/ir_actions.py", line 57, in run_action_code_multi
    res = super(ServerAction, self).run_action_code_multi(action, eval_context)
  File "/opt/odoo12/odoo/odoo/addons/base/models/ir_actions.py", line 434, in run_action_code_multi
    safe_eval(action.sudo().code.strip(), eval_context, mode="exec", nocopy=True)  # nocopy allows to return 'action'
  File "/opt/odoo12/odoo/odoo/tools/safe_eval.py", line 375, in safe_eval
    pycompat.reraise(ValueError, ValueError('%s: "%s" while evaluating\n%r' % (ustr(type(e)), ustr(e), expr)), exc_info[2])
  File "/opt/odoo12/odoo/odoo/tools/pycompat.py", line 86, in reraise
    raise value.with_traceback(tb)
  File "/opt/odoo12/odoo/odoo/tools/safe_eval.py", line 352, in safe_eval
    return unsafe_eval(c, globals_dict, locals_dict)
  File "", line 1, in <module>
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract.py", line 633, in cron_recurring_create_invoice
    invoices |= contracts_to_invoice._recurring_create_invoice(date_ref)
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract.py", line 618, in _recurring_create_invoice
    invoices_values = self._prepare_recurring_invoices_values(date_ref)
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract.py", line 587, in _prepare_recurring_invoices_values
    invoice_values=invoice_values,
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract_line.py", line 698, in _prepare_invoice_line
    name = self._insert_markers(dates[0], dates[1])
  File "/opt/odoo12/odoo-custom-addons/contract/models/contract_line.py", line 747, in _insert_markers
    name = name.replace('#END#', last_date_invoiced.strftime(date_format))
ValueError: <class 'AttributeError'>: "'bool' object has no attribute 'strftime'" while evaluating
'model.cron_recurring_create_invoice()'