Lately I had to reinstall a UAG server which is part of a two node array. The OS disk got corrupted somehow so a reinstall was necessary. When I wanted to rejoin the newly installed server to the UAG array I got the following error: “This server cannot join the array.” The procedure to add a UAG server to an array is explained in detail here: TechNet: Joining a server to an array

Whilst the procedure is very basic, it’s really important you don’t miss these items:

• Do not join a server to an array during Forefront UAG installation using the Getting Started Wizard.
• Ensure that the Forefront UAG Management console is closed on the array manager server

Also you might have to start some TMG services which are stopped during the initial (failed) attempt to join the array. This article is rather short and if you follow the documentation carefully you shouldn’t run into it. But somehow I did and I couldn’t find much for this error message. So here it is for future reference.