Increasing the memory limit did the trick.
I was suprised, however, that the csc compiler needs so much memory.
I had to increase to at least 600MB to succeed and now use a limit of 650MB for my C#-questions prototype.
Of course it is not only csc that uses the memory but the whole jobe toolchain.
At least csc needs much more memory than mcs does.
(Perhaps because it comes from Microsoft...)
Thanks for your help.