Module lock error when starting development server

Stuck. If anyone has ideas that would be brilliant.

Django 2.2, running on Ubuntu 18.04 in a venv

$ python manage.py runserver

Exception in thread django-main-thread:                                    
Traceback (most recent call last):                                
  File "/home/jonty/.virtualenvs/tdj_playground/lib/python3.6/site-packages/django/template/utils.py", line 66, in
 __getitem__                                                                  
    return self._engines[alias]                                                                                   KeyError: 'django'

and then another error, leading down finally to:

...

 File "<frozen importlib._bootstrap>", line 968, in _find_and_load
  File "<frozen importlib._bootstrap>", line 149, in __enter__
  File "<frozen importlib._bootstrap>", line 94, in acquire
_frozen_importlib._DeadlockError: deadlock detected by _ModuleLock('django.test.signals') at 139670284457464

Thanks in advance for any help.

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