Such error is generally shown when client code is taking an extensive amount of time to execute. There is a msdn article on similar error here
that might be interesting to you.
This being said, from the provided markup it would be hard to determine what exactly is causing the error. Would you share your full markup code with the client-side scripts and the related code behind? Would you also elaborate more on the steps that need to be taken in order for the error to appear? This would allow us to have better understanding of your scenario and try to find the root of the problem.
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