ASP.NET Ajax Error: Sys.WebForms.PageRequestManagerParserErrorException

I also got this error. The solution reported by "user1097991" solved it for a while (I was using not-serialized objects on viewstate)

But later the error returned again, now in a random fashion. After some search I got the answer: the viewstate was becoming too large and was been truncated. I disable some viewstates on grids and menus and the problem haven't shown again.

I found that my issue was related to a nul character being rendered in the databinding of a GridView. The expected length of the response wasn't matching the actual length of the response text which resulted in the error being thrown. Once I fixed the data in the database, I no longer got the error. The ultimate fix will be to sanitize the text getting rendered during the RowDataBound event.

Looking through the database, I couldn't see the bad data since SQL Server 2008 doesn't show the text if the nul character (Char(0)) is in the string. In the RowDataBound event of my GridView, I added code to throw an exception for any text that had special characters in it. This is how I found the record that contained the nul characters.

tl;dr - Check for nul characters in the rendered html.

Please also be aware that this can be caused by not properly html encoding what you may be rendering to the page through partial postbacks.

I had exactly the same error.

For me it was

<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

Missing in the httpModules section of web.config (.Net 3.5 app)

This error seems to may be related to many various things.

In our case the issue was caused by a rewriting proxy on the way. The rewrite modified the content of the update panel response. But this response also contains original size. The rewriting mechanism cannot know that few bytes of the response actually contains original response size and it should also be modified.

The update panel response starts like this:

1|#||4|30502|updatePanel|pnlUpdate| ...

The `30502` is original size of the content which is being updated. Rewriting engine modifies the output, but the size stays unchanged => parser error exception.

I don't see a way how to overcome this issue from the client side. We would need to know how exactly was the content modified and then somehow change the size in the response before UpdatePanel ClientScript starts processing it.

Update the ScriptMode to "Release"

<asp:ScriptManager ID="ScriptManager" runat="server" ScriptMode="Release"></asp:ScriptManager>