Unknown server tag 'asp:xxx'.

Hi *.*,

Its another common error seen after adding Ajax or by adding new web-config file to project.

Its happen when asp can't understand server tag !!!

Solution:

• Remove Ajax dll from bin folder add Ajax once more ('m not recommending this)
• Add tagprefix on webconfig.

How to Add tagprefix on webconfig?

<system.web>

<pages>

<controls>

<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

</controls>

</pages>

</system.web>

On this case make sure abt version you had added Other you will get version error
 

If u had any trouble just ask, Happy to help u :) 

Stay Tune... 

Have a nice day... 'N happy Coding :)