On Windows servers, the way special fonts (ie, Bar Codes, etc) are installed and used has changed due to the new PDF libraries included in Release 5. You will need to install the fonts on the server using Windows Control Panel and modify the Resources that use them. Unix/Linux installations do not require any changes. Please refer to the page for UsingCustomFonts for more information. This can be done on your existing 4.2 installation before installing 5.x.
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The way non-US characters are handled in the Java runtime has changed. If you use a non-US keyboard, we recommend adding -mapOptionKey=BACK_QUOTE to the command line or preferences file for the Desktop Client. See http://wiki.appx.com/wiki/bin/view/Main/ClientMapEndOptTab for more information.