This problem is often caused by a customized template applied to the Action. Try loading the Action without the template to see what happens.
To do that, take the Action URL and add "/t/0" before the "/p".
http://salsa4.salsalabs.com/o/51600/p/dia/action4/common/public/?action_KEY=123456 (The original URL.)
http://salsa4.salsalabs.com/o/51600/t/0/p/dia/action4/common/public/?action_KEY=123456 (The blank template URL.)
*NOTE: This "/t/0" trick does not work for Blind Targeted Actions (and, possibly, some Multi-Content Targeted Actions). For those, you have to actually access the Blind Targeted Action's setup, click Publishing Tools at the top, and assign a blank template to see the results.*
If, when loading the Action without your custom template applied, things begin to behave as expected, then your custom template is the culprit.
Investigating/correcting custom template code falls outside of the scope of our support, so our recommendation is to reach out to your web developer for assistance on getting the template code fixed up. Sometimes, just knowing that the behavior is being caused by the template, can lead you to the correct place for resolution. Here's an article showing where you can go to edit a template:
If no progress can be made via that method, we at Salsa do have a web developer on-staff, but there is generally a fee involved. Please reach out to us if you require further assistance with customizing template code, and we can pass your request over to an Account Manager for further discussion on getting our web developer involved.