Issues with Html.RenderAction after upgrade to 7.19

Hi,

We've just upgraded an EPiServer CMS 7 site to the latest version in the '7' branch (7.19).
We're experiencing issues with 'RenderAction' calls in our views which call into actions from ordinary 'Controllers'.
Such a call looks like 

@{Html.RenderAction("Index", "ShareMessage");}

What happens is that the 'Index' action on the current controller/context is called.
This results in a loop and eventually in a StackOverflowException.
It worked prior to the update.

I've tried a lot of possible routing configurations but with no success.

Any ideas are welcome.
Thanks!

Regards,

Robert.