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. 

Thanks Jessica.