Just my 2 Cents: I remember one weird issue where the Windows patch level made a difference.
This was specific to Windows 10. With one Windows 10 patch level Uniface could hang on startup when /pri or $ioprint was not set (i.e. I/O message level = 0) and with higher I/O message levels it worked. With a later Windows patch levels the problem disappeared again. The Uniface installation and the configuration of the application was the same in both cases, and the only difference was the patch level of Windows 10.
So, you might want to check if all the Windows updates are installed correctly and the machine is restarted (after the installation of the updates). I know from my own experience that Windows can behave rather erratic when it's not restarted after running Windows updates.
Apart from that, maybe the Windows Sysinternals utility Process Monitor (https://docs.microsoft.com/en-us/sysinternals/downloads/procmon) can shed some light where the Uniface process is hanging and if there's another process that interferes with the I/O of the application.
Hope this helps.
Regards,
Daniel