To reproduce the issue:

- Connect to a Citrix published desktop
- Open applications and disconnect the session
- Move to a separate workstation and open the same published desktop

The following error appears:

Logon Connect Failed
Error connecting to existing session for User (Id 3)
The operation completed successfully.
A new session will be created.

Set the colour depth of the display to the same for all workstations used to roam to e.g. 16 bit colour.

Also setting the published applications/desktop to the same resolution in the Presentation Server Console is preferable.