Sometimes Citrix application when launched via Program Neighborhood Agaent (PNAgent) shows the error message - Failed to Launch. The Server returned CharlotteAppHostUnreachable. The issue apparently is related to Zone Data Collector (ZDC), though I haven’t been able to find the reason accurately.
Usually server restart resolves the issue. If not, then to avoid downtime for the application, a quick workaround is to create an .ICA file for the application and provide that to users. Directly accessing the remote application via .ICA file would work even if the application shows that error message when launched via PNAgent. This way you can continue to troubleshoot the issue without impacting application users!