This problem occur whenever our json result set length exceeds default json MaxJsonLength property value. By default MaxJsonLength property will allow 102400 (100k) in case our string exceeding this length we will get JavaScriptSerializer error.
To solve this problem we need to increase default value of MaxJsonLength for that set MaxJsonLength property value in web.config file like as shown below
<configuration>
<system.web.extensions>
<scripting>
<webServices>
<jsonSerialization maxJsonLength="3000000"/>
</webServices>
</scripting>
</system.web.extensions>
</configuration>