@pgundlach I wonder what all is not permitted in beginning? In my testing (unrelated to speedata*), a disc node (with penalty:50) in the beginning of nodelist fed to tex.linebreak also results in the same error. I reached your page while searching for any information on this same error, and this is the only hit I got.
@pgundlach Btw, in my case, adding a local_par node in the very beginning (before disc with penalty) resolved this error. I wonder if that's it, and would cover all cases... In that case, its cleaner than having to check the id's and sequences of id's that hit this error.