Hello,
It would be very hard to determine the issue of the hang during the promotion, as the causes may be very different. Mostly it depends on the capability of the server to contact the Primary Domain controller due to a network communication issue, DNS configuration, etc.. Those would be the first things to check.
On the other hand, to help you find more details or specific errors, you can access the OS, please collect the following logging file to verify the Dcpromo hangs issue, DcpromoUI.log and Dcpromo.log, and review the contents for the lasts lines to see if they throw more information:
The DcpromoUI.log file contains a detailed progress report of user selections and operations through the progression of the Dcpromo wizard. Its default location is the %SystemRoot%\Debug folder.
The Dcpromo.log file is created by using AD installation wizard and records a summary of critical operations. Its default location is the %SystemRoot%\Debug folder.
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