oarchive oa(ofs);
        oa << (*this);

And this one was right:

        std::ofstream ofs(filename.c_str(), std::ios::binary);
        boost::archive::binary_oarchive oa(ofs);
        oa << (*this);

The consequence of the first code is a "stream_error" throw which doesn't say much about the problem.
Instead, an exception should be thrown earlier on if the file is not in binary mode.

thanks alot to all the boost contributors,
Frank