I was not able to reproduce the problem.
Is there a specific directory or specific way you open a directory that causes the problem? Do I understand right that it's not happening on every change of remote directory (i.e. the Back button sometime/most of the time works)?

I'm using WinSCP 5.5.1 (build 3970) in Windows 7 with Explorer interface (single panel). Changing directory of a remote unix host (SFTP connection), the blue toolbar button arrows (prev/next) become gray, to reactivate them I click with right button mouse in the list after the last file (free space) and hover the go|prev or go|next menu item. It break again after few directory change. This is a bit annoying.

I saw this bug few year ago from version to version and never fixed.