I had a look at it.
On macOS 10.15.7 Calibre 5.5.0 I get the same behaviour. Changing the ' Match by' in the selection of the application doesn't change a thing.
No error in the KM log.
In the macOS system log I get this error: (com.apple.xpc.launchd.oneshot.0x1000004a.calibre[13223]): Service exited with abnormal code: 2
Perhaps this helps someone else with further troubleshooting.
I installed it using Homebrew. And it is installed at /Applications/calibre.app.
I did try this again:
reinstalled calibre using Homebrew
created a new macro using Activate action
ran the macro
The first time I ran the macro I got the message about calibre being downloaded from the internet. I choose open. And then.... nothing and the KM action time out message.
I'm not the OP and don't use calibre myself so this is just out of curiosity: why do you expect it in /Applications/Optional/ ? And does KM expect it there?
I use calibre and so I had a quick look and I also suffer from the problem I initiate Calibre from the KM macro (posted above) and nothing happens for 20 seconds and then KM times out.
So I thought I'd initiate Calibre differently like this:
and when I now run the macro, Calibre runs properly!
I don't know why this makes a difference. Instead of Default Application you can also specify Finder (as per the action's help information) and that works for me too.
That's where the macro from the first post points to.
I would expect it to be in /Applications/ and I would think Keyboard Maestro would too. If it was in /Applications/ and then was moved to another folder, that might explain why it wouldn't launch.
Would that cause KM to throw an error if the app wasn’t where it should be, as opposed to just timing out? My installation of calibre is definitely in /Applications but I get this odd behaviour too.
I have plenty of apps in /Applications/Optional and I can start them all with KM. I just have to select the app in the Activate menu while it is running, but then it can be launched via Activate even when it is not running