server execution error in wpf chatting application based on signalr and .Net 4.5.1

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.NET

WPF

errors

execution

.NET4.5

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I developed a chatting application in wpf based on SignalR and .Net 4.5.1. But sometimes it gives server execution error. The situation is that when we login it happens normally but when is start session it give server execution error and the interesting thing is that it server execution error on specific system again and again and on some system it run perfectly. I tried it by formatting system and delete registry but the server execution error comes again. I don't understand why it is happening. Please help me.