Hi Matthias,
For any error the first step is to find out the error details, as in basic troubleshooting.
However, the bad news is that I can no longer produce a good build for .NET 4.0. .NET 4.5 replaces .NET 4.0 and now that I have installed the latest updates to Visual Studio, if I try to compile a build for .NET 4.0 it really uses .NET 4.5 because it replaced 4.0.
Also 4.5 is 4.0 runtime, so even if .NET 4.5 is installed it will say 4.0... under Administration > System Information. So the only good way to know if you have .NET 4.5 hosting is to ask your host and they can see under Control > Panel Programs what version of .NET is installed. 4.5 has been out for 2 years now and is like a service pak for 4.0.
If you really do only have 4.0 hosting and not 4.5 then the only advice I have is to use mojoPortal 2.4.0.3. The 4.0 files I provided for 2.4.0.4 turned out to really not be 4.0 but 4.5 and I have no way to fix it going forward because I do not have any machine that does not already have 4.5 installed and I can no longer produce a good build for 4.0 on any of my machines. Therefore going forward I can only provide builds for 4.5. I have removed our download for 4.0 for mojoportal 2.4.0.4 because it does not work reliably and gives strange errors that seem to indicate it is really for .NET 4.5
I'm sorry for this problem but there is nothing I can do to solve it, really Microsoft created this problem by making 4.5 a replacement for 4.0 but it has breaking changes.
The only solutions are use 4.5 hosting with the latest version of mojoPortal or use an older version of mojoPortal (2.4.0.3) if you don't have 4.5 hosting.
Sorry for the bad news,
Joe