How to fix Allnet ALL3073V2 'Remote control functionality not activated!'

Problem:

When trying to access an ALL3073V2 device via the xml/jsonswitch.php API (e.g. http://192.168.0.100/xml/jsonswitch.php?id=1), you get the following error message:

{
  "error": "777994",
  "text": "Remote control functionality not activated!"
}

Solution:

You need to activate the remote control functionality via the webinterface.

Open the webinterface, then open Configuration -> Servers and users:

ALL3073V2 Servers and users

Now click on the Access control tab:

ALL3073V2 Access control

After that, first click on the Activate Remote Control Enable button. Once you’ve done that, you can now click on the Save inputs button:

ALL3073V2 Activate remote control

The device will then confirm the change:

ALL3073V2 confirmation

You can now access the JSON API immediately, no reboot etc is required.