After probably the installation of an outdated program messsed up some libraries, the window "Missing Api-ms-win-core-crt-l1-1-0.dll" popped when starting particular programs.
As solution the installation of "Microsoft Visual C++ 2015 Redistributable (vc_redist.x64.exe and vcredist_x86.exe)" is recommended. However, this installation permanently failed.
Also the installation of Visual Studio 2015 itself failed, because the necesssary KB2999226 also faild to install. Manual installation of KB2999226
wusa.exe Windows6.1-KB2999226-x64.msu /extract:XXXX
DISM.exe /Online /Add-Package /PackagePath:XXXX\Windows6.1-KB2999226-x64.cab
seemed to work (package successfuly added) but did not help. Visual Studio 2015 and vc_redist.x64.exe failed to install.