Generally I would recommend only printing from an exported PDF. Even if you're not going to distribute your document you can use the PDF as a final "proof" before printing. Think of it as getting a second opinion, and if you do come to distribute your document you'll already know that it's okay.
Also, unless you need one of the special functions of the Print Preview you might want to look at using the Preview Mode instead (use the eye icon near the bottom right of the main window). Personally I never use either Print or Print Preview in Scribus.
As for why the line goes missing (and comes back), that's a bit of a head scratcher. Sometimes it can happen if "disable export" (Properties Palette / X,Y,Z / Printer Icon) has been accidentally pressed. A similar thing can happen to whole layers if the checkbox for that layer under the "eye" icon in the Layers Manager has been unchecked.
Unfortunately there's no way of finding out which objects aren't exportable without checking each one. And there is no "globally make everything exportable" function as a quick fix when you don't know what could have happened.
(One way of making a line go missing, of course, is to give it the same colour as the background or set its transparency to 100%, but you're not doing that. Or you can set the Opacity of the layer to 0% but that's probably not what's going on either.)
Maybe someone else has a better idea of what's going on (especially if they can open your file and reproduce the problem). It could be something very simple that I'm missing.
As for which version to use, it's normally recommended that people use the latest stable version unless they really need some of the new features. Personally I'd get the document looking as correct as I could using the stable version (with placeholders where necessary), then open it in the new version - saving under a new name so the previous version doesn't get overwritten - and continuing from there. This all depends on your personal level of risk vs. reward though. Many people use 1.5.1 on a day-to-day basis without any problem so it's an individual choice. (Your 1.5.1 installation is nearly two months old so it might be worth updating to get the latest code anyway.)