If you were trying to upgrade via an upgrade module I would suggest attempting a manual upgrade. Not all servers are alike, which makes automating the upgrade difficult and sometimes buggy.
A manual upgrade (uploading the files via FTP, or unzipping the files using a tool like SSH) will give you fewer problems, although it can be a slower process. Just overwrite your old files with the latest version files, and then navigate to /install/upgrade.php to perform an upgrade. I suggest overwriting rather than deleting and uploading because users often lose customized files like /libs/dbconnect.php or the /avatars directory, which should be left in place when upgrading.
The biggest thing that you should consider is a template upgrade. If you follow our release threads, we try to outline any template changes made between versions so that you will know what files need to be updated.
You may also want to double check your /languages/lang_english.conf (or other language) file when your upgrade is complete to make sure that all of the latest variables have been added to the bottom of the file. For example if you are upgrading to 1.1.1 you should see the following at the bottom of your language file:
Code:
// End 1.1.1 Language File