pgAdmin4 error "Failed to launch the application server, server thread exiting."

I'm getting this error on startup of pgAdmin4 on Kubuntu 18.04:

An error occurred initialising the application server:
Failed to launch the application server, server thread exiting.

In the following window ("Configuration") I have these default values:

Python Path:
/usr/pgadmin4/lib/python2.7;/usr/pgadmin4/lib/python2.7/site-packages
Application Path:
/usr/pgadmin4/web

Since I don't have a /usr/pgadmin4/ directory, all of them are wrong.

See https://stackoverflow.com/questions/43968302/failed-to-launch-application-server-server-thread-existing-on-windows for the same error on Windows

For me, when I run pgadmin like Thomas Landauer says, I got this error:

$ sudo python3 /usr/share/pgadmin4/web/pgAdmin4.py
Traceback (most recent call last):
  File "/usr/share/pgadmin4/web/pgAdmin4.py", line 58, in <module>
    exec(open(file_quote(setupfile), 'r').read())
  File "<string>", line 47, in <module>
  File "/usr/share/pgadmin4/web/pgadmin/setup/data_directory.py", line 29, in create_app_data_directory
    _create_directory_if_not_exists(config.SESSION_DB_PATH)
  File "/usr/share/pgadmin4/web/pgadmin/setup/data_directory.py", line 15, in _create_directory_if_not_exists
    os.mkdir(_path)
FileNotFoundError: [Errno 2] No such file or directory: '/var/cache/pgadmin/sessions'

The error says what is the problem, so I created the folders:

$ sudo mkdir -p /var/cache/pgadmin/sessions

And after restarting pgAdmin4, it first asked for user creation, then worked.

The problem seems to be that I have python2 and python3 installed. Solution (workaround) that works for me:

1. Locate your pgAdmin4.py. In my case, it's /usr/share/pgadmin4/web/pgAdmin4.py
2. Start it in a terminal like this: sudo python3 /usr/share/pgadmin4/web/pgAdmin4.py

If you have a better solution, please let us know :-)

I opened a ticket for this: https://redmine.postgresql.org/issues/3737

Forget about the prebuilt package (apt). It is broken in so many ways (Python path, PIP path, filesystem permission) while not showing much debug info.

Use Python wheel instead. At least it will should you what exactly is broken, chances are you need to

• install a few modules (mako)
• run with sudo (or fix file permission issue manually)
• check logs ls .pg* -lh

Probably not the optimal way but here is how I got it to work

python3.7 -m pip install --user https://ftp.postgresql.org/pub/pgadmin/pgadmin4/v4.5/pip/pgadmin4-4.5-py2.py3-none-any.whl

sudo python3.7 ~/.local/lib/python3.7/site-packages/pgadmin4/pgAdmin4.py