There was a forum post for a similar issue over a year ago. That was just after a Cox Webmail update and there were still a few issues. But, I don't think the OP posted their solution. The first thing I would do is to delete myemail.cox.net cookies and close your browser. Note: you don't need to delete all your cookies. When you access webmail the next time, your webmail cookies will be re-created. I can't guarantee that will fix it, but it won't hurt anything.
The post from last year:
Strange new screen when trying to access COX webmail regardless of device type - Internet - Internet Forum - Cox Support Forums