pgadmin4 : postgresql application server could not be contacted.

I found the same issue when upgrading to pgAdmin 4 (v1.6). On Windows I found that clearing out the content inside C:\Users\%USERNAME%\AppData\Roaming\pgAdmin\sessions folder fixed the issue for me. I believe it was attempting to use the sessions from the prior version and was failing. I know the question was marked as answered, but downgrading may not always be an option.

Note: AppData\Roaming\pgAdmin is a hidden folder.

We got the same issue. so no any change in the file. but start pgAdmin 4 using administrator.

The following step. 1. right click pgAdmin 4 icon 2. select "Run As Administrator"