Trouble with Membership sign ups

I seem to be having a serious problem with the Membership plugin sign ups.

I have been using this plugin for almost two years and have had various problems, but this is driving me crazy, maybe someone can look at the page and see why it is happening.

Some people seem to be able to sign up and complete their payment page correctly.

While others are able to fill in their information, but are unable to go to the payment page. Instead they see this message in a box at the bottom of the page:

You are not currently logged in. Please login to view your subscriptions.

I am using the Business Portfolio child theme and use 2checkout as my gateway.

In addition, if someone puts in their info but does not complete the sign up they are unable to reach any page to complete the payment at a later date by signing in with their user name and password. I thought that should take them to the payment page.

Can someone please take a look and see what is wrong? This is a large membership site and I am losing members like crazy. My only alternative is to sign them up manually and have them send me a check, which just does not work for me.

Thanks in advance for any help.