Bernie, you can report problems with any Scribus version in the Mantis Issue Tracker
http://bugs.scribus.net but you'll need to signup for an account - which is easy to do. (A quick search for "thick lines" didn't show any outstanding issues but that's no guarantee that no-one else has the same problem.)
Before you do that though, try resetting the preferences back to the defaults by using the "Defaults" button on the Preferences dialog. I know you'll need to go back and alter stuff again but it might fix other issues you could have in the future too.
Remember, 1.5.0svn isn't a production release and you can't open documents saved by it in previous versions. So if you come to an unsurmountable problem with 1.5.0svn you can't just go back to 1.4.x without having to re-do the document.
P.S. I've got 1.5.0svn installed on my Mac - along with 1.4.2 - and I've not noticed any issues with thick lines though there was a problem with keyboard shortcuts which was fixed by this kind of reset.