i have recently installed pgadmin4 onto my laptop and when I launch the application, it just gets stuck on the loading. I had a look at the logs and this is what I see:

The logs
2020-11-14 00:22:46: Checking for system tray...
2020-11-14 00:22:46: Starting pgAdmin4 server...
2020-11-14 00:22:46: Creating server object, port:64222, key:2a079549-63da-44d2-8931-efa9de3a847f, logfile:C:/Users/yonis/AppData/Local/pgadmin4.d41d8cd98f00b204e9800998ecf8427e.log
2020-11-14 00:22:46: Python Path: C:/Program Files/PostgreSQL/13/pgAdmin 4/venv/Lib/site-packages;C:/Program Files/PostgreSQL/13/pgAdmin 4/venv/DLLs;C:/Program Files/PostgreSQL/13/pgAdmin 4/venv/Lib
2020-11-14 00:22:46: Python Home: C:/Program Files/PostgreSQL/13/pgAdmin 4/venv
2020-11-14 00:22:46: Initializing Python...
2020-11-14 00:22:46: Python initialized.
2020-11-14 00:22:46: Adding new additional path elements
2020-11-14 00:22:46: Redirecting stderr...
2020-11-14 00:22:46: stderr redirected successfully.
2020-11-14 00:22:46: Initializing server...
2020-11-14 00:22:46: Webapp Path: C:/Program Files/PostgreSQL/13/pgAdmin 4/web/pgAdmin4.py
2020-11-14 00:22:46: Server initialized, starting server thread...
2020-11-14 00:22:46: Open the application code and run it.
2020-11-14 00:22:46: Set the port number, key and force SERVER_MODE off
2020-11-14 00:22:46: PyRun_SimpleFile launching application server...
2020-11-14 00:22:47: Application Server URL: http://127.0.0.1:64222/?key=2a079549-63da-44d2-8931-efa9de3a847f
2020-11-14 00:22:47: The server should be up. Attempting to connect and get a response.
2020-11-14 00:22:53: Attempt to connect one more time in case of a long network timeout while looping
2020-11-14 00:22:53: Everything works fine, successfully started pgAdmin4.
You can add the following lines to the " config_distro.py " and restart the pgAdmin 4 server:
import mimetypes mimetypes.add_type('text/javascript', '.js')
See this answer for the long explanation.
Version 4.29 is available. Install it instead of 4.28. It works.
https://www.postgresql.org/ftp/pgadmin/pgadmin4/v4.29/windows/
The previous answers mentioning config.py and config_distro.py are almost there but according to https://www.pgadmin.org/docs/pgadmin4/4.28/config_py.html perhaps the best file to use is config_local.py. This is applied after the config.py and config_distro.py settings are applied. As this file is not distributed with the installation, it won't be overwritten on an upgrade.
In Windows, depending on your version number and installation location, these can be located in the "%programfiles%\PostgreSQL\13\pgAdmin 4\web" folder for example.
So to fix this issue, create a file config_local.py in the same location as config.py and insert the following lines:
import mimetypes
mimetypes.add_type('text/javascript', '.js')
Finally, restart the postgresql server and you should be good as gold!
Same problem with v4.28 get stuck loading screen. and v4.27 cannot open, it crashes. now I downgrade to v4.26 it's ok
Have a look at following link: https://www.postgresql-archive.org/pg-Admin-4-v4-28-Errors-on-launch-td6162407.html I think this is the fix you need
Hi All We were not able to reproduce this issue on any of our machines during pre-release testing for v4.28. Then after some R&D, we are able to reproduce this issue. Cause of the problem: if value of "HKEY_CLASSES_ROOT.js\Content Type" is set to "text/plain" then issue is reproducible. In any javascript file, the content type should be “text/javascript” in the response header but in the above registry setting it is "text/plain", so the browser won't allow it. Solution: Open the registry and changed the value of "HKEY_CLASSES_ROOT.js\Content Type" to "text/javascript".
Update:
PgAdmin4 5.0 is out now, you can check it out here. It seems this problem is no longer an issue. take a look at my screen shot below:

My registry value is still "text/plain" and my PgAdmin works as below shows:
So, if you still have this problem, try upgrade to PgAdmin4 5.0.
On the official FAQ a solution is outlined here (& why it happens): https://www.pgadmin.org/faq/#12
I also had the same issue, it was working normally and suddenly stopped to work. I tried lots of different approaches, updating version, uninstall/install, and others, but nothing changed, just an HTML screen with 6 links not working and a message saying PGadmin was loading.... finally I tried to turn off my adblocker, and it worked. Probably it is some issue regarding an update from the adblocker or PGAdmin
with according with this fix explain here
If someone have this problem on windows,
first the reason of that is the value of "HKEY_CLASSES_ROOT\.js\Content Type" is set to "text/plain" on registry manager and windows not accept it.
Now change that value to "text/javascript" it all.
Shut down the pgAdmin server to see those changes.
Now all can work fine, you don't have to uninstall anything or downgrade anything.