Well, after spending hours on this issue, this is apparently what worked:
File.Copy(File.DirAssets, "seas_18.se1", File.DirInternal, "seas_18.se1")
then set the path to File.DirInternal.
I don't know why I needed to copy the file, then set the path to File.DirInternal. In my other application, I didn't have to copy the text files from File.DirAssets - I just read them from there and all was well. But I can't just read these - I must copy, then set the path to Dir.DirInternal.
I'm confused, still.