Apparently that's a known issue on KDE. The workaround is to
From https://forum.devolutions.net/topics/34476/remote-desktop-manager-does-not-worl "When stuck at the registration screen, click the cancel button, that should get the splash screen to go away. From there, you can simply go to the Help tab and register from there."
Pinned Comments
tgm4883 commented on 2023-07-14 00:18 (UTC)
Support for this software (Remote Desktop Manager) should happen on their forum: https://forum.devolutions.net/product/rdm-linux
Issues with the packaging should be reported here.