The none-working iPad is able to discover the windows device with other apps, and manually adding IP in spacedesk always stuck at IP validation.
What do you mean by “windows device with other apps”?
..so it cannot discover the spacedesk server, right?
..so the non working iPad has the latest iOS beta version.. is it possible to downgrade it to non beta version e.g. 14.7?
For further analysis, could you please send us the dxdiag info of your primary machine?
Just run dxdiag.exe on your primary machine, click “Save all information” and send us the output file.