We've had this exact same problem with 2 clients. We don't just make the same changes to both of them, so definitely an MS issue. Which they seem to have fixed in the incoming update 'now'.
When you navigate to the Website and form tab on Event and change the Custom registration form URL? toggle, it errors both times. Users can choose to ignore the error as it's just an error.
MS Support has struggled to work out the issue - have we changed anything, are we using OOB Forms, etc.. Handlers switched on etc.. yes, formcontext passed in, yes...
Eventually twigged that the issue is with the Event Handler, so whilst it is on the onLoad event and correctly called we've added it to the Tab onChange event.
As a workaround, we did the following
EventMgmt.CrmWebResource.Handlers.customEventUrlHandler added this to the Tab onChange event and then the error disappeared.
Will see what MS support comes back with, eventually.
Hope this helps someone now and in the future our clients are on crm4, with Marketing.
