The error message clearly shows that it's not a credentials issue but a configuration issue.
Check the RC Status of the machine in question in the DSM Explorer\Computers and Users\All Computers\<Machine>.
If it shows as "Listening (Local Security)", make the following changes to the policy applied to the machine(s):
Go to Control Panel\Configuration\Configuration Policy\Default Computer Policy\DSM\Remote Control\Host\Managed
Change the Centralized Security to True
If the machine shows as just "Listening", check the effective settings for the machine. This can be done by right clicking on the machine in the DSM explorer, going to Remote Control then effective settings. Enter the user you are attempting to connect with and hit enter. This will then show you what permissions that user has for that machine. If the user is not allowed to perform the connection type being attempted, then grant the user permissions. This can be handled under the group that the computer is located in, under [Group Details]> Remote Control Permissions.
If they are allowed to perform the connection type but it is still getting rejected, please open an issue with support.