The phenomenon: you log in successfully and see the administration back-end. On the first navigation (clicking an admin menu item, navigating to another page, saving the page content from the editor etc.) you get logged out!
The problem: the login cookies couldn't be set. Either the browser doesn't accept cookies or the server doesn't send them. Probably the latter is the case, but please double check, that your browser does accept cookies.
Well, let's presume the server doesn't send the cookies. This is probably caused by some premature output. So check, that
- your site doesn't contain a BOM in any of the PHP files: see http://cmsimpleforum.com/viewtopic.php?f=4&t=3583
If there's a BOM, you have to find the respective file. In the cmsimple/ folder check config.php, languages/en.php and languages/enconfig.php (or the respective files for your language), cms.php and login.php with an editor, which tells you the encoding of the files. It should be UTF-8 without BOM -- otherwise convert it to this encoding. If you're using a version before CMSimple_XH 1.2 the encoding is probably different -- but it should never be UTF-8 with BOM.
- your PHP files don't contain any characters outside of <?php ... ?>
If there are superfluous characters, delete them.
- there's no error output of PHP (warnings, notices) on top of the screen
If there is error output of PHP check your PHP ini settings from CMSimple_XH's back-end: error_reporting should be 0 and display_errors should be Off. If the settings are different, you either have enabled XH debug mode or the server configuration forbids to change these settings dynamically. The latter has to be changed by your provider.
Ideally there shouldn't be any error output at all independent of the respective settings, so report it to the developers. OTOH: most of these warnings and notices are quite harmless.
PS: Make sure, you don't use a subdomain which contains an underscore.