Hi Christian,
The problem with a javascript error "Sys is not defined" has come up a few times recently and when it happens it does cause login failure and the solution you posted is correct and solves the problem.
But to clarify,this should in general not be needed in Web.config under .NET 4 and it never was in the Web.config file that we ship in the .NET 4 packages of mojoPortal as far as I know. I have checked our source code repository where I can see the complete file history and it has never been removed from the file and has never been in the file. I also looked in our old packages for mojoPortal 2.3.9.4 and it does not exist in the web.config file for the .NET 4 packages of that version. I do not have that in any of my site installations including this site.
Something similar was required for .NET 3.5 and has been our web.config for .NET 3.5 packages and still is there in our current package for 3.5, maybe that is where you saw it in versions 2.3.9.4.
In .NET 4 they moved this kind of thing to machine.config so it is not needed in Web.config, and in fact it cannot be in both places or it causes an error. For example if I add it in Web.config on my installation it causes this error:
"Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'ScriptResourceIntegrated-4.0'"
So whenever this problem of sys is undefined comes up for .NET 4 it seems to me like something is not configured correctly in the installation of .NET 4 on the server. When it does come up the site can be fixed by adding this to Web.config but it should not be needed on a correctly installed and configured .NET 4 installation. In fact if you put this in Web.config and later the server configuration gets corrected it will crash the site with the above error. Myself I would try re-installing .NET 4.5 framework instead of putting that in Web.config.
If you create a new .NET 4 web application project in Visual Studio the web.config file it generates also does not have this. So it is not something missing in our Web.config file and not something we should add there by default.
Best,
Joe