Custom Action doesn't work from inside a Lookup

Custom Action doesn't work from inside a Lookup

I have setup a simple Custom Action that uses OpenURL to open our Client Form in a new window.
This works just fine when clicked from the clients report. 

I have an additional report with a list of tasks related to our clients.
This report lists tasks from a form that has a Lookup field that looks up the client.
The report shows the client's name, and when I click on the name, it opens up the client's record, so I can see his details.
So far so good, and everything is working as it should.

However, the record view has the custom action button in it, as it should, but when this button is clicked, the application just hangs.
In fact, I've checked numerous options, and no matter what the action (or the function it executes) does, the button does absolutely nothing. It just hangs the system, and forces me to refresh the page.

I want to be able to click this button, in order to edit the client's details.
Is that possible?