I get this:
Warning: fopen(/home/mysite/public_html/serendipity_config_local.inc.php): failed to open stream: Permission denied in /home/mysite/public_html/include/functions_installer.inc.php on line 36
Your Serendipity installation is now upgraded to version 0.9.1
Nothing has been upgraded however.
Problem with upgrade
This is a permissions problem. Serendipity wasn't able to write its config file into the serendipity directory. What you need to do depends on your environment, but if you want a surefire solution, change that directory to have 777 permissions, and the same for the exising serendipity_config_local.inc.php file.