If you are upgrading from Geeklog v2.1.3 or older please make sure your 3rd party Geeklog plugins are updated to the latest version and support at least Geeklog v2.2.0. Plugins are now required to use the function COM_createHTMLDocument. This replaces the depreciated functions COM_siteHeader and COM_siteFooter. If you are unsure if a plugin supports Geeklog v2.2.0 then either disable it before you upgrade Geeklog, or uninstall it.
Also, there have been some changes with blocks and topics between Geeklog 2.0.0 and 1.8.2sr1 so not all plugins may work with 2.1.0 or higher unless they have been updated. If you are upgrading your site that is 2.0.0 or older make sure all the plugins you use are compatible with Geeklog 2.1.0 or higher.
This documentation is a result of community action. Everyone is invited to sign up and participate. If you see an omission you can fill it in. If you see a mistake, correct it. If you see where things could be better organized, change it. If you have a note about a particular configuration, add it. In other words: We need your help to make it better!