there are couple of reasons for this error.
if you have antivirus installed on the server, then make sure it is not blocking those dll to deploy.
make sure SharePoint timer and Admin services running on all servers
May be restart the time service and admin service on all sever, some time timer service lock the files on the server.
also check if you package ...
To get a more detailed error message directly in the browser, you can:
Edit the web.config files located at c:\inetpub\wwwroot\wss\VirtualDirectories\<port_number> AND C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\LAYOUTS\ modify the parameters you have indicated in that web.config file:
. customErrors mode must be ...
Are you sure the web.config is referencing to the right dll?
So to check goto the assembly and find the dll file Telerik.Web.UI.dll.
See the name matches the safe control, same witht the public key and version!!!
From what i can see you have Version=2010.3.1215.20 but isnt that for later versions not wss 3.0? shouldnt it be somthing like 2008.1.512.20 ...
You'll just need to add a <%@ Register %> tag for your Telerik component (assuming you're building a visual webpart) and to make sure it is installed on GAC.
That <SafeControls /> entry is meant to register which webparts are trusted by your web application. Telerik doesn't contains webparts, so it doesn't need to be listed there.
The webpart or any controls not visible in edit mode may be due to EditModePanel. You can check the wiki page layout in SharePoint designer and see if the webpart zone is in EditModePanel. The EditModePanel has property named PageDisplayMode which is Edit by default.
If PageDisplayMode=Display the content of EditModePanel appear only when the page is in ...