members log in page takes user to “oops” page

The site I am trying to manage is for a nonprofit organization. To be a member, one has to join and pay either a one year or 3 year membership. We then give them access to members only areas of the site. Lately I have been receiving a couple of notices each day of new WordPress users registering on the site.
there are a couple of different notices formats, one just shows me the new persons username and email address, another notice gives me a lot more information about the new user.
I have contacted some of these people and they claim not to have tried to register for access to the site.
If I just click on the members log in page It says the page does not exist.

I have noticed that when logged out of the site, if I click on a members only area of the site I am taken to a page to log in and below that is a New User Registration form. Like this one http://www.sigdalslag.org/members-area/emigrant-list

I suspect that this form popping up on the members only area is the culprit. Would that be correct and how do I eliminate that registration form from appearing. If I am logged in as the administrator, I do not see that form

Can someone help me please

The page I need help with: [log in to see the link]