I just updated my VS2012 to update 4 and my Telerik controls to v.2013.3.114.35. I also have JustCode v2013.3.1119.1 installed. Since then, my aspx intellisense no longer works for the telerik controls and I also get an "unrecognized tag prefix or device filter 'telerik'" message on each telerik control (tag.jpg). I have my telerik controls registered in my web.config file instead of on each aspx page (there are 300+ pages). I can see the controls in design view, the project compiles and runs okay. I do get intellisense for the asp controls (aspx) and for both asp and telerik in the code behind (vb). I have uninstalled the telerik controls and reinstalled, all to no avail. I have disabled/enabled JustCode. When I am inside a telerik control (aspx) and hit the space bar (intellisense should kick in here) all I get is a ( "db" Markup snippet for a data-bind attribute ) (snippet.jpg) in place of the intellisense.

Does anyone have any idea of how to fix this.

One thing I forgot to mention is this solution/project was created in vs2008 and upgraded to vs2012.

Hello Dennis,

This error is showed by VS and not by JustCode that is why nothing is improved when JustCode is disabled.
I made some research and found the following articles that could help:

1. Link1
   
2. Link2

Unfortunately, I was unable to reproduce the issue and test the solutions. 
Thanks.

Regards,
Zdravko
Telerik

I found several posts across the internet with users having the same problem. One post showed changes that were made to the web.config file, so I made the following change to the web config file and all intellisense works. Not sure why it works but it does.

Old settings in the configuration/configsections

Changes: