I managed to fix the compile errors by doing the following:

• download a previous source version of dnn
• copy the dir Libriries\Components\AJAX to from the 4.9.3 to the 4.9.4 dir
• add "Imports DotNetNuke.Common.Utility" to the head of your global.aspx

Now, i´ve no idea it the AJAX component is fully compatible with 4.9.4 but since i didn´t see any update note i guess it is.

You don't need to import part of the old version. Just remove in the DotNetNuke.Library.vbproj file
the copy statement for the System.Web.Extension. This file is in your GAC and doesn't need to be in the bin dir.

Together with the added import statement as mentioned and making the web directory of your solution a virtual directory in your IIS with full permissions for ASPNET and Networkservice should fix the problem

After Lots of googling i found

http://forums.asp.net/p/1055356/1547872.aspx

It says to solve this problem by going to

http://ajax.asp.net/default.aspx?tabid=47&subtabid=471

ajax.asp.net/default.aspx

Download zipfile including web.system.extensions.dll
en zet die in DNN/Website/Library/Controls/AJAX/bin/