Unfortunately, it is hard for us to tell you why you are getting the error without knowing what you are submitted in that field. However, you shouldn't share that information for security reasons. So essentially all we can do is speculate as to why you are getting the error. All I know is I submitted a ticket yesterday and had no issue entering my email.
The new staff contact forum isn't part of wikia tho, its an external service called Zendesk, but either way you should be able to email firstname.lastname@example.org your zendesk ticket and it should create a ticket for you that way.