Same problem.
Server Windows 10 21H1, spacedesk driver v 1.0.17
Client android 4.4.2, spacedesk android app v 0.9.79
The android application gives a fatal error when trying to connect to the server. Other devices with more modern android have no problems.
I have tested with the HTML5 viewer and it works fine, although the performance is worse. The problem occurs only with the app.
Regards