Windows: Protocol problem Error code = 5, suberror = 0

Follow these steps if you're seeing the error 'Protocol Error = 5, sub error = 0' when logging into your Windows desktop:

As of Jump Desktop Connect 6.0, you are no longer allowed to login into a different user's desktop session on Windows if you're authenticating using a non-administrator Windows user account. This is an intentional breaking change due to security concerns with allowing standard users to log into desktop sessions other than their own. Only Administrators are allowed to log into other user's desktop sessions. If you want to switch between users while connected to the machine, make sure you're logging in using an administrator account. 

Other alternatives: 

1. Use the new Jump Desktop Connect Screen Sharing feature with supports password-less logins: Jump Desktop 8.0+ now supports the 'Ask to Share Screen' feature which prompts the remote console user if they want to start a screen sharing session with you. Alternatively the remote user can also open up 'Jump Desktop Connect' and share the 'screen sharing' link with you - click that to connect.  
2. If you already know the user name and password of the local account you want to log into,  simply remove the saved password from the Fluid connection and then connect to the remote computer. When Jump prompts you for your 'Windows Credentials', make sure you use the user account credentials you want to login as. Alternatively you could also duplicate the connection for the computer in Jump and save credentials for each user you want to connect as.

Related link about Screen Sharing: Whats New in Jump Desktop Connect 6.0

Updated Aug 21st 2018: Jump Desktop Connect 6.0.72 did not allow Administrators to connect to other user's sessions. In 6.0.75 (released Aug 21st) we relaxed the requirements and now Fluid will allow Administrators to connect to other user's sessions (thanks for all the feedback).