What does the 'Port in use' error mean?
When running the Automator, it runs by default on port 7070, but keep in mind that you do have the option to change this port to any other number.
If this port (7070 or whichever port you configured) is already in use, then the Automator will give you the error that the port is already in use, meaning it is not available for the Automator itself.
'Port in use' on MacOS
'Port in use' on Windows
How to resolve the 'Port in use' error?
Seeing that this error can have multiple causes, there are multiple solutions.
An Automator instance is already running
In this case, it means that other Automator instance is already taking up the port.
The solution here is quite straightforward:
Windows:
Check the system tray (bottom right corner) and check for the Automator 'C' icon. If multiple are present, make sure to right-click all of them to quit all instances. Next, restart a single Automator.
Optionally, check the Task Manager for any running Automator processes and make sure to quit them before restarting a single Automator.
MacOS:
Check the menu bar (top right corner) and check for the Automator 'C' icon. If multiple are present, make sure to right-click all of them to quit all instances. Next, restart a single Automator.
Check the Activity Monitor application for any remaining Automator processes and make sure to quit them before restarting a single Automator.
Once all the (extra) processes/applications have been terminated, restarting a single Automator should now work as intended.
Another application (other than the Automator) running on your computer is using this port
This case potentially needs a little bit deeper digging, as you might need to find out which other application that is.
Alternatively, the easier solution might be to change the port on which the Automator is running.
Go to the Automator application itself (not the GUI in the browser)
Change the port from 7070 into another (available) port
Confirm the change by clicking 'Change port'
Restart the Automator
When changing the port of the Automator, do keep in mind that you will now access the GUI (in the browser) via this new port.
In case you use the Prompter via the Automator, you will have to use the new port to access the prompter as well.
If you use the Cuez Media Player, then here as well you will be using the new port to access the CMP in the browser.