Simple Image Gallery Pro will not produce errors of that sort (trust me on this).
RS Form Pro and Akeeba Admin & Backup are widely used extensions so I don't think these are to blame either.

You can try disabling plugins one by one to see which is causing the issue.

I am having this same problem with the latest version of Joomla and the latest version of K2. The problem is that K2 is not logging in when the admins log into the site. So we need to find a way to get k2 to log in when the user logs in.

@Douglas Musgrove

K2 does not have it's own login system. Once user signs in in Joomla! the only thing that K2 checks is his permissions. So as i wrote in the previous three topics you posted the same message turn on system debugging to see where the error is raised from. Also stop posting duplicate messages.

FYI my client moved the site to a new host and the problem disappeared :dry:

Obviously this was a server setup issue. If you are using Nginx as a web server then you need to apply the correct rewrite rules docs.joomla.org/Nginx .