Try to restart the Remote Management service on the mac computer. On the mac, go in System Preferences - Sharing and uncheck/check the Remote Management service.
Activating logging on the mac may help the Devolutions support team resolving the issue. In order to activate the logging server side, execute the following command in Terminal:
sudo defaults write /Library/Preferences/com.apple.RemoteManagement ARDCollectLogs -bool YES
Once this is done, you can try to connect normally using Remote Desktop Manager. The log will then be located in the /tmp folder on the Mac computer.
To turn off logging, execute this command:
sudo defaults write /Library/Preferences/com.apple.RemoteManagement ARDCollectLogs -bool NO