I am afraid that the provided information was not enough for me to reproduce the issue on my side, because of many missing references (like classes, database, etc.). With the current information I could only guess what might be causing the problem and propose the following solutions:
- Try using RadAjaxManager instead an update panel. You can take a look at the approach used in this live demo, also loading the content from a data base: Using Custom Content Provider
- Disable the page validation by setting the ValidateRequest property of the <%@ Page directive to "false"
If non of the suggestions above helps in solving the case, I will need to replicate and examine the issue locally. You can see how to prepare a sample fully runnable project reproducing the problem in the following blog post: Isolating a problem in a sample project
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