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Regarding your question, there are two possible causes for this error. The first is that the server-side machine is not configured properly to allow the client access to either the computer or the particular VI.
The second possibility is that two computers are attempting to access the same VI on the same remote computer at the same time on a system with only a single seat license.
I would suggest following the tutorial linked below closely, and then also ensuring that only one machine is using the remote front panel at a time.
Remote Panels in LabVIEWScott Yellig
Applications Engineer
National Instruments