(I'm hoping the following is a well-known problem with a simple solution. If not, then I'm posting this so the SilverStripe developers know about the problem.)
(1) install works fine
(2) after install, go to /admin page; EVERYTHING WORKS, can modify pages, etc.; "publish" works, changes show up on pages (indicating database is connected and working)
(3) close browser, come back later; earlier page changes still show up correctly; login works, shows "Welcome back [first name]" correctly; BUT when try to reach /admin page, gives the following error:
Premature end of script headers: main.php
Error 500: Script Execution Failure
The error is NOT caused by:
(a) login problems (username, password are correct)
(b) total failure of database (since at least some data is being stored, such as the page changes)
(c) file permissions (all permissions are set to 755 -- or is this a problem?)
(d) changes to scripts (I have made no changes of any kind; it's a fresh install)
(e) cookies (are allowed, and do appear in the browser's cookie list)
The webhost is called "200MB Host", using the "free" service. The limitations of the free service do not seem to be the problem; there should be enough memory, the PHP and MySQL versions appear to meet the requirements.
The browser is Firefox 3.0.6; get same problem with Internet Explorer.
Have tried this numerous times with different configurations (such as different base directory, subdomain, themes, etc.); always installs fine then gives the same problem.
Remember, everything installs and works properly the first time! Just can't return to /admin after closing the browser/logging off and then logging back in.