that's a method that is no longer used in the core since 5.0 - it's implementation looks like this:

 [Obsolete("Deprecated in 5.0. Shouldn't need to be used any more.  ContainerObjects (IContainerControl implementations) have a property ModuleControl.")]

        public static PortalModuleBase GetPortalModuleBase(UserControl control)

        {

            PortalModuleBase moduleControl = null;

            Panel panel;

            if (control is SkinObjectBase)

            {

                panel = (Panel) control.Parent.FindControl("ModuleContent");

            }

            else

            {

                panel = (Panel) control.FindControl("ModuleContent");

            }

            if (panel != null)

            {

                try

                {

                    moduleControl = (PortalModuleBase) panel.Controls[1];

                }

                catch

                {

     //check if it is nested within an UpdatePanel

                    try

                    {

                        moduleControl = (PortalModuleBase) panel.Controls[0].Controls[0].Controls[1];

                    }

                    catch (Exception exc)

                    {

                        Exceptions.LogException(exc);

                    }

                }

            }

            return moduleControl ?? (new PortalModuleBase {ModuleConfiguration = new ModuleInfo()});

        }

As it's not been used in years there is a chance that it has an error that only affects certain sites e.g. sites that have some skin/container/module that explicitly used that's API method - it's hard for me to say and harder still to test - that said it does look like any possible error would come from a container not having "ModuleContent" within it's name - I tested on a local install where I was working with a custom module ("cathaltestinputfilter") and mine does have a server control with that - I recommend you do a view source on the failing page(s) and see if that is the case with your install

<div id="dnn_ctr415_ModuleContent" class="DNNModuleContent ModcathaltestinputfilterC">