Comment # 1 on bug 1166681 from Erik Martre

The dependency issue seems to have been solved since a couple of days, with an
upgrade to version 4.19 and python 3.8, but there is a new bug with the last
build, which can be easily fixed. 

pgAdmin4 won't launch, and will display a message box : "fatal error : the
server could not be contacted". When we do not click on this message and
right-click on the systray icon, then "View log..." we see that the last lines
are:

  " File
"/usr/lib/python3.8/site-packages/pgadmin4-web/pgadmin/browser/__init__.py",
line 32, in <module>
        from flask_security.views import _security, _commit,
default_render_json, _ctx
ImportError: cannot import name 'default_render_json' from
'flask_security.views'
(/usr/lib/python3.8/site-packages/flask_security/views.py) "

If you edit /usr/lib/python3.8/site-packages/flask_security/views.py, you'll
see that there is actually no definition for default_render_json, but there's
one for _render_json.

So you go to
/usr/lib/python3.8/site-packages/pgadmin4-web/pgadmin/browser/__init__.py (I
hope I made no typo), you edit it as su and you simply replace on line 32
default_render_json with _render_json. Register the file and now pgAdmin4 will
launch.