Automation

Yes, the EX and QFX both support REST. Here is a Feature Explorer page for REST. 

For the EX4300, you would need Junos OS 20.2R1. The QFXs are listed as well. I have used REST on both the EX and QFX and haven't had any issues with it, but that doesn't help you with the current situation. 

I would try: 
- https instead of http (because, why not?)
- keeping it super simple, if you are not already. Meaning, no VC or things like sending REST through a port that isn't the management port, and checking if it works.
- restart/reboot if not in production (again, because why not?)

jessica@ex3400-24p> show configuration system services rest 
http {
    port 3000;
}
enable-explorer;

jessica@ex3400-24p> show system connections | grep 3000        
tcp6       0      0  *.3000                                        *.*                                           LISTEN
tcp4       0      0  *.3000                                        *.*                                           LISTEN

% curl http://X.X.X.X:3000/rpc/get-system-information -u "jessica:Password"
<system-information format="xml">
<hardware-model>ex3400-24p</hardware-model>
<os-name>junos</os-name>
<os-version>21.4R3.15</os-version>
<serial-number>############</serial-number>
<host-name>ex3400-24p</host-name>
</system-information>

I get the expected results from CLI and cURL. Before you try to fix this, would like to see what the error message is when you try cURL from the shell.