'Sys' is undefined error on webpage

Hi,
When I open my webpage locally on my computer, the webpage does not through any error but from the server, I get those:
Message: ASP.NET Ajax client-side framework failed to load.
Message: 'Sys' is undefined
Line: 678
Message: 'Sys' is undefined
Line: 2542
In the web.config file I changed:
<compilation debug="true"> to <compilation debug="false">
wich solved the first error(ASP.NET Ajax client-side framework failed to load.)
 
1. I wonder if that is okay to solve that error like that?
2. Then I wonder for the 'Sys' is undefined, what is causing that error. I googled and could solve the first error but this Sys error it seems to have countless of solutions that have not worked for me, so I might wonder what can cause this ...