Umm... Well I don't know specifically for this, I use know there was a recent blog post on 5.6.5 related to some JavaScript problems and issues in DNN. In this case though I am not sure why and I would need to test/research your specific Client Side Event more. We don't have any one else repotting this and its a pretty basic/standard feature of client side events so it could be related to how its being rendered, what is the exact field type for this field, is the field hidden at one point, are there other JavaScript errors (you can test the exact error that is happening in FireBug for FireFox).
What you can do is:
1. Enable FireBug for FireFox (after downloading and installing it)
2. Enable the "Console" for FireBug
3. Enable Debug Mode for Dynamic Forms, this will spit out the exact Dynamic Forms client side event that the module is using).
4. Finally - Copy/Paste the client side event that is rendered from debug mode into the FireBug Console and run the JavaScript --- What error is returned?
When all else fails (and actually I don't think this specifically is related to 5.6.5 now that I read it again), you can pick up Premium Support at
http://www.datasprings.com/PremiumSupport or hopefully another customer has some feedback if they are running into a similar issue. Client Side Events/JavaScript are not a part of our standard support agreement but I am sure we can hammer it out quickly if you need to with the Premium Support route.
Thanks!
-Chad