Hi Matthias,
mojoportal-core.sln has no features except Html content feature. If you want to do a build without the features like Blog, Forums, etc, you can do it with this solution. However for that to work must do a clean checkout and never build the main solution mojoportal.sln becuase it has all the features and once you build it copies all the files for installing the other features up to the Web project. So later when you build just the core it throws erros because the features were previously installed. So the only way to get a build without features is clean checkout in a new folder, use a new db and only compile the mojoportal-core.sln or the custom copy.
I recommend not to use mojoportal-core.sln unless you DO NOT want any of the features.
If you do want all the features then you should copy mojoportal.sln to create your custom .sln file. Then you will have all the included features and can add projects for your custom features.
Hope it helps,
Joe