In case that login is failed even though using the correct user ID and password, it may cause that PostgresSQL has not been running properly.
Please try the following procedure and check if this issue is solved.
1. Open the "Service" list in "Administration Menu" on a terminal which Vex is installed.
(Example for Windows 10)
Right click the "start menu", then select "Computer management">"Service and application">"Service".
2. Right click "vex_postgres" and select "property".
3. When the status is "Stopped", change it to "Start".
* When the "Startup type" is not "Automatic", set to "Automatic" also.
When the PostgresSQL status has already "Start" or still there is this issue even doing procedure, it seems to have a different cause.
In this case, please contact Vex support team and provide Vex log file.